Teleological and Teleonomic: A New Analysis
By Ernst Mayr

…::::::::::::::::::::::::From: Mayr, Ernst (1974) Boston Studies in the Philosophy of Science. 
…::::::::::::::::::::::::::::::::::::Volume XIV, pages 91 -117.

Teleological language is frequently used in biology in order to make statements about the functions of organs, about physiological processes, and about the behavior and actions of species and individuals. Such language is characterized by the use of the words 'function', 'purpose', and 'goal', as well as by statements that something exists or is done 'in order to'. Typical statements of this sort are 'It is one of the functions of the kidneys to eliminate the end products of protein metabolism', or 'Birds migrate to warm climates in order to escape the low temperatures and food shortages of winter'. In spite of the long-standing misgivings of physical scientists, philosophers, and logicians, many biologists have continued to insist not only that such teleological statements are objective and free of metaphysical content, but also that they express something important which is lost when teleological language is eliminated from such statements. Recent reviews of the problem in the philosophical literature (Nagel, 1961; Beckner, 1969; Hull, 1973; to cite only a few of a large selection of such publications), concede the legitimacy of some teleological statements but still display considerable divergence of opinion as to the actual meaning of the word 'teleological' and the relations between teleology and causality.

This confusion is nothing new and goes back at least as far as Aristotle, who invoked final causes not only for individual life processes (such as development from the egg to the adult) but also for the universe as a whole. To him, as a biologist, the form-giving of the specific life process was the primary paradigm of a finalistic process, but for his epigones the order of the universe and the trend toward its perfection became completely dominant. The existence of a form-giving, finalistic principle in the universe was rightly rejected by Bacon and Descartes, but this, they thought, necessitated the eradication of any and all teleological language, even for biological processes, such as growth and behavior, or in the discussion of adaptive structures. 

The history of the biological sciences from the 17th to the 19th centuries is characterized by a constant battle between extreme mechanists, who explained everything purely in terms of movements and forces, and their opponents who often went to the opposite extreme of vitalism. After vitalism had been completely routed by the beginning of the 20th century, biologists could afford to be less self-conscious in their language and, as Pittendrigh (1958) has expressed it, were again willing to say 'a turtle came ashore to lay her eggs', instead of saying 'she came ashore and laid her eggs'. There is now complete consensus among biologists that the teleological phrasing of such a statement does not imply any conflict with physico-chemical causality. 

Yet, the very fact that teleological statements have again become respectable has helped to bring out uncertainties. The vast literature on teleoloy is eloquent evidence for the unusual difficulties connected with this subject. This impression is reinforced when one finds how often various authors dealing with this subject have reached opposite conclusions (e.g. Braithwaite, 1954; Beckner, 1969; Canfield, 1966; Hull, 1973; Nagel, 1961). They differ from each other in multiple ways, but most importantly in answering the question: What kind of teleological statements are legitimate and what others are not? Or, what is the relation between Darwin and teleology? David Hull (1973) has recently stated "evolutionary theory did away with teleology and that is that", yet a few years earlier McLeod (1957) had pronounced "what is most challenging about Darwin, is his reintroduction of purpose into the natural world". Obviously, the two authors must mean very different things.

Purely logical analysis helped remarkably little to clear up the confusion. What finally produced a breakthrough in our thinking about teleology was the introduction of new concepts from the fields of cybernetics and new terminologies from the language of information theory. The result was the development of a new teleological language, which claims to be able to take advantage of the heuristic merits of teleological phraseology without being vulnerable to the traditional objections.

1. TRADITIONAL OBJECTIONS TO THE USE OF TELEOLOGICAL LANGUAGE

Criticism of the use of teleological language is traditionally based on one or several of the following objections. In order to be acceptable, teleological language must be immune to these objections.

1. Teleological Statements and Explanations Imply the Endorsement of Unverifiable Theological or Metaphysical Doctrines in Science

This criticism was indeed valid in former times, as for instance when natural theology operated extensively with a strictly metaphysical teleology. Physiological processes, adaptations to the environment, and all forms of seemingly purposive behavior tended to be interpreted as being due to non-material vital forces. This interpretation was widely accepted among Greek philosophers, including Aristotle, who discerned an active soul everywhere in nature. Bergson's (1910) élan vital and Driesch's (1909) Entelechie are relatively recent examples of such metaphysical teleology. Contemporary philosophers reject such teleology almost unanimously. Likewise, the employment of teleological language among modern biologists does not imply adoption of such metaphysical concepts (see below).

2. The Belief that Acceptance of Explanations for Biological Phenomena that Are not Equally Applicable to Inanimate Nature Constitutes Rejection of a Physico-Chemical Explanation

Ever since the age of Galileo and Newton it has been the endeavor of the 'natural scientists' to explain everything in nature in terms of the laws of physics. To accept special explanations for teleological phenomena in living organisms implied for these critics a capitulation to mysticism and a belief in the supernatural. They ignored the fact that nothing exists in inanimate nature (except for man-made machines) which corresponds to DNA programs or to goal-directed activities. As a matter of fract, the acceptance of a teleonomic explanation (see below) is in no way in conflict with the laws of physics and chemistry. It is neither in opposition to a causal interpretation, nor does it imply an acceptance of supernatural forces in any way whatsoever.

3. The Assumption that Future Goals were the Cause of Current Events Seemed in Complete Conflict with any concept of Causality

Braithwaite (1954) state the conflict as follows:

•	In a [normal] causal explanation the explicandum is explained in terms of a cause which either precedes it or is simultaneous with it; in a teleological explanation the explicandum is explained as being causally related either to a particular goal in the future or to a biological end which is as much future as present or past.

This is why some logicians up to the present distinguish between causal explanations and teleological explanations.

4. Teleological Language Seemed to Represent Objectionable Anthropomorphism

The use of words like 'purposive' or 'goal-directed' seemed to imply the transfer of human qualities, such as intent, purpose, planning, deliberation, or consciousness to organic structures and to sub-human forms of life.

Intentional, purposeful human behavior is, almost by definition, teleological. Yet I shall exclude it from further discussion because the words 'intentional' or 'consciously premeditated', which are usually used in connection with such behavior, endanger us with getting involved in complex controversies over psychological theory, even though much of human behavior does not differ in kind from animal behavior. The latter, although usually described in terms of stimulus and response, is also highly 'intentional', as when a predator stalks his prey or when the prey flees from the pursuing predator. Yet, seemingly 'purposive', that is goal-directed behavior in animals can be discussed and analyzed in operationally definable terms, without recourse to anthropomorphic terms like 'intentional' or 'consciously'.

As a result of these and other objections teleological explanations were widely believed to be a form of obscurantism, an evasion of the need for a causal explanation. Indeed, some authors went so far as to make statements such as 'Teleological notions are among the main obstacles to theory formation in biology' (Lagerspetz, 1959: 65). Yet, biologists insisted in continuing to use teleological language.

The teleological dilemma, then consists in the fact that numerous and seemingly weighty objections against the use of teleological language have been raised by various critics, and yet biologists have insisted that they would lose a great deal, methodologically and heuristically, if they were prevent from using such language. It is my endeavor to resolve this dilemma by a new analysis, and particularly by a new classification of the various phenomena that have been traditionally designated as 'teleological'.

2. THE HETEROGENEITY OF TELEOLOGICAL PHENOMENA

One of the greatest shortcomings of most recent discussions of the teleology problem has been the heterogeneity of the phenomena designated as 'teleological' by different authors. To me it would seem quite futile to arrive at rigorous definitions until the medley of phenomena, designated as 'teleological', is separated into more or less homogeneous classes. To accomplish this objective, will be my first task.

Furthermore, it only confuses the issue, when a discussion of teleology is mingled with that of such extraneous problems as vitalism, holism, or reductionism. Teleological statements and phenomena can be analyzed without reference to major philosophical systems.

By and large all the phenomena that have been designated in the literature as teleological, can be grouped into three classes:

(1) Unidirectional evolutionary sequences (progressionism, orthogenesis).

(2) Seemingly or genuinely goal-directed processes.

(3) Teleological systems.

The ensuing discussion will serve to bring out the great differences between these three classes of phenomena.

1. Unidirectional evolutionary sequences (progressionism, orthogenesis, etc.)

Already with Aristotle and other Greek philosophers, but increasingly so in the 18th century, there was a belief in an upward or forward progression in the arrangement of natural objects. This was expressed most concretely in the concept of the scala naturae, the scale of perfection (Lovejoy, 1936). Originally conceived as something static (or even descending owing to a process of degradation), the Ladder of Perfection was temporalized in the 18th century and merged almost unnoticeably into evolutionary theories such as that of Lamarck. Progressionist theories were proposed in two somewhat different forms. The steady advance toward perfection was either directed by a supernatural force (a wise creator) or, rather vaguely, by a built-in drive toward perfection. During the flowering of Natural Theology the 'interventionist' concept dominated, but after 1859 it was replaced by so-called orthogenetic theories widely held by biologists and philosophers (see Lagerspetz, 1959: 11-12 for a short survey). Simpson (1949) refuted the possibility of orthogenesis with particularly decisive arguments. Actually, as Weissmann had said long ago (1909), the principle of natural selection solves the origin of progressive adaptation without any recourse to goal-determining forces.

It is somewhat surprising how many philosophers, physical scientists, and occasionally even biologists, still flirt with the concept of teleological determination of evolution. Teilhard de Chardin's (1955) entire dogma is built on such a teleology and so are, as Monod (1971) has stressed quite rightly, almost all of the more important ideologies of the past and present. Even some serious evolutionists play, in my opinion rather dangerously, with teleological language. For instance Ayala (1970:11) says,

•	the overall process of evolution cannot be said to be teleological in the sense of directed towards the production of specified DNA codes of information, i.e. organisms. But it is my contention that it can be said to be teleological in the sense of being directed toward the production of DNA codes of information which improve the reproductive fitness of a population in the environments were it lives. The process of evolution can also be said to be teleological in that it has the potentiality of producing end-directed DNA codes of information, and has in fact resulted in teleologically oriented structures, patterns of behavior, and regulated mechanisms.

To me this seems a serious misinterpretation. If 'teleological' means anything it means 'goal-directed'. Yet, natural selection is strictly an a posteriori process which rewards current success but never sets up future goals. No one realized this better than Darwin who reminded himself "never to use the words higher or lower." Natural selection rewards past events, that is the production of successful recombinations of genes, but it does not plan for the future. This is, precisely, what gives evolution by natural selection its flexibility. With the environment changing incessantly, natural selection – in contradistinction to orthogenesis – never commits itself to a future goal. Natural selection is never goal oriented. It is misleading and quite inadmissible to designate such broadly generalized concepts as survival or reproductive success as definite and specified goals.

The same objection can be raised against similar arguments presented by Waddington (1968: 55-56). Like so many7 other developmental biologists, he is forever looking for analogies between ontogeny and evolution. "I have for some years been urging that quasi-finalistic types of explanations are called for in the theory of evolution as well as in that of development." Natural selection "in itself suffices to determine, to a certain degree, the nature of the end towards which evolution will proceed, it must result in an increase in the efficiency of the biosystem as a whole in finding ways of reproducing itself." He refers here to completely generalized processes, rather than to specific goals. It is rather easy to demonstrate how ludicrous the conclusions are which one reaches by over-extending the concept of goal-direction. For instance, one might say that it is the purpose of every individual to die because this is the end of every individual, or that it is the goal of every evolutionary line to become extinct because that is what has happened to 99.9% of all evolutionary lines that have ever existed. Indeed, one would be forced to consider as teleological even the second law of thermodynamics.

One of Darwin's greatest contributions was to have made it clear that teleonomic processes involving only a single individual are of an entirely different nature from evolutionary changes. The latter are controlled by the interplay of the production of variants (new genotypes) and their sorting out by natural selection, a process which is quite decidedly not directed toward a specified distant end. A discussion of legitimately teleological phenomena would be futile unless evolutionary processes are eliminated from consideration.

2. Seemingly or Genuinely Goal-Directed Processes

Nature (organic and inanimate) abounds in processes and activities that lead to an end. Some authors seem to believe that all such terminating processes are of one kind and 'finalistic' in the same manner and to the same degree. Taylor (1950), for instance, if I understand him correctly, claims that all forms of active behavior are of the same kind and that there is no fundamental difference between one kind of movement or purposive action and any other. Waddington (1968) gives a definition of his term 'quadi-finalistic' as requiring "that the end state of the process is determined by its properties at the beginning."

Further study indicates, however, that the class of 'end-directed processes' is composed of two entirely different kinds of phenomena. These two types of phenomena may be characterized as follows:

A. Teleomatic processes in inanimate nature. Many movements of inanimate objects as well as physico-chemical processes are the simple consequences of natural laws. For instance, gravity provides the end state for a rock which I drop into a well. It will reach its end-state when it has come to rest on the bottom. A red-hot piece of iron reaches its 'end-state' when its temperature and that of its environment are equal. All objects of the physical world are endowed with the capacity to change their state and these changes follow natural laws. They are 'end-directed' only in a passive, automatic way, regulated by external forces or conditions. Since the end state of such inanimate objects is automatically achieved such changes might be designated as teleomatic. All teleomatic processes come to an end when the potential is used up (as in the cooling of a heated piece of iron) or when the process is topped by encountering an external impediment (as a falling stone hitting the ground). Teleomatic processes simply follow natural laws, i.e. lead to a result consequential to concomitant physical forces, and the reaching of their end state is not controlled by a built-in program. The law of gravity and the second law of thermodynamics are among the natural laws which most frequently govern teleomatic processes. 

B. Teleonomic processes in living nature. Seemingly goal-directed behavior in organisms is of an entirely different nature from telomatic processes. Goal-directed 'behavior' (in the widest sense of this word) is extremely widespread in the organic world; for instance, most activity connected with migration, food-getting, courtship, ontogeny, and all phases of reproduction is characterized by such goal orientation. The occurrence of goal-directed processes is perhaps the most characteristic feature of the world of living organisms.

The definition of the term teleonomic. For the last 15 years or so the teleonomic has been used increasingly often for goal-directed processes in organisms.  I proposed in 1961 the following definition for this term: "It would seem useful to restrict the term teleonomic rigidly to systems operating on the basis of a program, a code of information" (Mayr, 1961). Although I used the term 'system' in this definition, I have since become convinced that it permits a better operational definition to consider certain activities, processes (like growth) and active behaviors as the most characteristic illustrations of teleonomic phenomena. I therefore modify my definition, as follows: A teleonomic process or behavior is one which owes its goal-directedness to the operation of a program. The term teleonomic implies goal-direction. This, in turn, implies a dynamic process rather than a static condition, as represented by a system. The combination of 'teleonomic' with the term system is, thus, rather incongruent (see below).

All teleonomic behavior is characterized by two components. It is guided by a 'program' and it depends on the existence of some end point, goal, or terminus which is foreseen in the program that regulates the behavior. This endpoint might be a structure, a physiological function, the attainment of a new geographical position, or a 'consummatory' (Craig, 1918) act in behavior. Each particular program is the result of natural selection, constantly adjusted by the selective value of the achieved endpoint.

My definition of 'teleonomic' has been labeled by Hull (1973) as a 'historical definition'. Such a designation is rather misleading. Although the genetic program (as well as its individually acquired components) originated in the past, this history is completely irrelevant for the functional analysis of a given teleonomic process. Whether this program had originated through a lucky macromutation (as Richard Goldschmidt had conceived possible) or through a slow process of gradual selection, or even through individual learning or conditioning as in open programs, is quite immaterial for the class of a process as 'teleonomic'. On the other hand, a process that does not have a programmed end, does not qualify to be designated as teleonomic (see below for a discussion of the concept 'program').

All teleonomic processes are facilitated by specifically selected executive structures. The fleeing of a deer from a predatory carnivore is facilitated by the existence of superlative sense organs and the proper development of muscles and other components of the locomotory apparatus. The proper performing of teleonomic processes at the molecular level is made possible by highly specific properties of complex macromolecules. It would stultify the definition of 'teleonomic' if the appropriateness of these facilitating executive structures were made part of it. On the other had it is in the nature of a teleonomic program that it does not induce a simple unfolding of some completely preformed Gestalt, but that it always controls a more or less complex process which must allow for internal and external disturbances. Teleonomic processes during ontogenetic development, for instance, are constantly in danger of being derailed even if only temporarily. There exist innumerable feedback devices to prevent this or to correct it. Waddington (1957) has quite rightly called attention to the frequency and importance of such homeostatic devices which virtually guarantee the appropriate canalization of development.

We owe a great debt of gratitude to Rosenblueth et al. (1943) for their endeavor to find a new solution for the explanation of teleological phenomena in organisms. They correctly identified two aspects of such phenomena, (1) that they are seemingly purposeful being directed toward a goal, and (2) that they consist of active behavior. The background of these authors was in the newly developing field of cybernetics and it is only natural that they should have stressed the fact that goal directed behavior is characterized by mechanisms which correct errors committed during the goal-seeking. They considered the negative feedback loops of such behavior as its most characteristic aspect and stated "teleological behavior thus becomes synonymous with behavior controlled by negative feedback." This statement emphasizes important aspects of teleological behavior, yet it misses the crucial point: The truly characteristic aspect of goal-seeking behavior is not that mechanisms exist which improve the precision with which a goal is reached, but rather that mechanisms exist which initiate, i.e. 'cause' this goal-seeking behavior. It is not the thermostat that determines the temperature of a house, but the person who sets the thermostat. It is not the torpedo which determines toward what ship it will be shot and at what time, but the naval officer who releases the torpedo. Negative feebacks only improve the precision of goal-seeking, but do not determine it. Feedback devices are only executive mechanisms that operate during the translation of a program.

Therefore it places the emphasis on the wrong point to define teleonomic processes in terms of the presence of feedback devices. They are mediators of the program, but as far as the basic principle of goal achievement is concerned, they are of minor consequence.

3. Recent Usages of the Term Teleonomic

The term 'teleonomic' was introduced into the literature by Pittendrigh (1958:394) in the following paragraph:

•	Today the concept of adaptation is beginning to enjoy an improved respectability for several reasons: it is seen as less than perfect; natural selection is better understood; and the engineer-physicist in building end-seeking automata has sanctified the use of teleological jargon. It seems unfortunate that the term 'teleology' should be resurrected and, as I think, abused in this way. The biologists' long-standing confusion would be more fully removed is all end-directed systems were described by some other term, like 'teleonomic', in order to emphasize that the recognition and description of end-directedness does not carry a commitment to Aristotelian teleology as an efficient [sic] casual principle.

It is evident that Pittendrigh had the same phenomena in mind as I do,1 even though his definition is rather vague and his placing the term 'teleonomic' in opposition to Aristotle's 'teleology' is unfortunate. As we shall see below, most of Aristotle's references to end-directed processes refer precisely to the same things which Pittendrigh and I would call teleonomic (see also Delbrück, 1971).

Other recent usages of the term that differ from my own definition are the following. B. Davis (1962), believing that the term denotes 'the development of valuable structures and mechanisms' as the result of natural selection, uses the term virtually as synonymous with adaptiveness. The same is largely true for Simpson (1958: 520-521) who sees in 'teleonomic' the description for a system or structure which is the product of evolution and of selective advantage:

•	The words 'finalistic' and 'teleological' have, however, had an unfortunate history in philosophy which makes them totally unsuitable for use in modern biology. They have too often been used to mean that evolution as a whole has a predetermined goal, or that the utility of organization in general is with respect to man or to some supernatural scheme of things. Thus these terms may implicitly negate rather than express the biological conclusion that organization in organisms is with respect to utility to each separate species at the time when it occurs, and not with respect to any other species or any future time. In emphasis of this point of view, Pittendrigh [above] suggests that the new coinage 'teleonomy' be substituted for the debased currency of teleology. 

Monod (1971) likewise deals with teleonomy as if the word simply meant adaptation. It is not surprising therefore that Monod considers teleonomy "to be a profoundly ambiguous concept." Furthermore, says Monod, all functional adaptations are "so many aspects or fragments of a unique primary project which is the preservation and multiplication of the species." He finally completes the confusion by choosing "to define the essential teleonomic project as consisting in the transmission from generation to generation of the invariant content characteristic of the species. All these sturctures, all the performances, all the activities contributing to the success of the essential project will hence be called teleonomic."

What Monod calls 'teleonomic' I would designate as of 'selective value'. Under these circumstances it is not surprising when Ayala (1970) claims that the term 'teleonomy' had been introduced into the philosophical literature in order 'to explain adaptation in nature as the result of natural selection'. If this were indeed true, and it is true of Simpson's and Davis's cited definitions, the term would be quite unnecessary. Actually, there is nothing in my 1961 account which would support this interpretation, and I know of no other term that would define a goal-directed activity or behavior that is controlled by a program. Even though Pittendrigh's discussion of 'teleonomic' rather confused the issue and has led to the subsequent misinterpretations, he evidently had in mind the same processes and phenomena which I denoted as teleonomic. It would seem well worthwhile to retain the term in the more rigorous definition, which I have now given. 

3. THE MEANING OF THE WORD PROGRAM

The key word in my definition of 'teleonomic' is the term 'program'. Someone might claim that the difficulties of an acceptable definition for teleological language in biology had simply been transferred to the term 'program'. This is not a legitimate objection, because it neglects to recognize that, regardless of its particular definition, a program is (1) something material, and (2) exists prior to the initiation of the teleonomic process. Hence, it is consistent with a causal explanation.

Nevertheless, it must be admitted that the concept 'program' is so new that the diversity of meanings of this term has not yet been fully explored. The term is taken from the language of information theory. A computer may act purposefully when given appropriate programmed instructions. Tentatively program might be defined as coded or prearranged information that controls a process (or behavior) leading it toward a given end. As Raven (1960) has remarked correctly the program contains not only the blueprint but also the instructions of how to use the information of the blueprint. In the case of a computer program or of the DNA of the cell nucleus the program is completely separated from the executive machinery. In the case of most man-made automata the program is part of the total machinery. 

My definition of program is deliberately chosen in such a way as to avoid drawing a line between seemingly 'purposive' behavior in organisms and man-made machines. The simplest program is perhaps the weight inserted into loaded dice or attached to a 'fixed' number wheel so that they are likely to come to rest on a given number. A clock is constructed and programmed in such a way as to strike at the full hour. Any machine which is programmed to carry out goal-directed activities is capable of doing thi 'mechanically'.

The programs which control teleonomic process in organisms are either entirely laid down in the DNA of the genotype ('closed programs') or are constituted in such a way that they can incorporate additional information ('open programs') (Mayr, 1964), acquired through learning, conditioning or through other experiences. Most behavior, particularly in higher organisms, is controlled by such open programs. Let me illustrate this with an example. A young cuckoo or cowbird, although raised in the nest of foster parents, associates after fledging with other members of its own species and pairs with them when reaching reproductive age. The information controlling all this is part of the inherited DNA program, a closed program. On the other hand, members of certain species of ducks and geese or estrildid finches lack such inborn information concerning the recognition of members of their own species. It is subsequently inserted into their open program by a special 'learning' process called imprinting. It is still unknown whether such an acquired program is coded chemically (e.g. RNA) or through a highly specific pattern of synapses or both. Once the open program is filled in, it is equivalent to an originally closed program in its control of teleonomic behavior. 

Open programs are particularly suitable to demonstrate the fact that the mode of acquisition of a program is an entirely different matter from the teleonomic nature of the behavior controlled by the program. Nothing could be more purposive, more teleonomic than much of the escape behavior in many prey species (in birds and mammals). Yet, in many cases the knowledge of which animals are dangerous predators is learned by the young who have an open program for this type of information. In other words this particular information was not acquired through selection and yet is it clearly in part responsible for teleonomic behavior. Many of the teleonomic components of the reproductive behavior (including mate selection) of species which are imprinted for mate recognition is likewise only partially the result of selection. The history of the acquisition of a program, therefore, can not be made part of the definition of teleonomic. 

The origin of a program is quite irrelevant for the definition. It can be the product of evolution, as are all genetic programs, or it can be the acquired information of an open program, or it can be a man-made device. Anything that does not lead to what is at least in principle a predictable goal does not qualify as a program. Even though the future evolution of a species is being set severe limits by its current gene pool, its course is largely controlled by the changing constellation of selection pressures and is therefore not predictable. It is not programmed inside the contemporary gene pool. 

The entire concept of a program of information is so new that it has received little attention from philosophers and logicians. My tentative analysis may, therefore, require considerable revision when subjected to further scrutiny. 

How does the program operate? The philosopher may be willing to accept the assertion of the biologist that a program directs a given teleonomic behavior, but he would also like to know how the program performs this function. Alas, all the biologist can tell him is the study of the operation of programs is the most difficult area of biology. For instance, the translation of the genetic program into growth processes and into the differentiation of cells, tissues, and organs is at the present time the most challenging problem of developmental biology. The number of qualitatively different cells in a higher organism almost surely exceeds one billion. Even though all (or most) have the same gene complement they differ from each other owing to differences in the repression and derepression of individual gene loci and owing to differences in their cellular environment. It hardly needs stressing how complex the genetic program must be, to be able to give the appropriate signals to each cell lineage in order to provide it with the mixture of molecules which it needs in order to carry out its assigned tasks. 

Similar problems arise in the analysis of goal-directed behavior. The number of ways in which a program may control a goal-directed behavior activity is legion. It differs from species to species. Sometimes the program is largely acquired by experience, in other cases it may be almost completely genetically fixed. Sometimes the behavior consists of a series of steps, each of which serves as reinforcement for the ensuing steps, in other cases the behavior, once initiated, seems to run its full course without need for any further input. Feedback loops are sometimes important but their presence cannot be demonstrated in other kinds of behavior. Again, as in developmental biology, much of the contemporary research in behavioral biology is devoted to the nature and the operation of the behavior sequences (Hinde and Stevenson, 1970). Almost any statement one might make is apt to be challenged by one or the other school of psychologists and geneticists. It is, however, safe to state that the translation of programs into teleonomic behavior is greatly affected both by sensory inputs and by internal physiological (largely hormonal) states. 

4. TELEOLOGICAL SYSTEMS

The word 'teleological', in the philosophical literature, is particularly often combined with the term 'system'. Is it justified to speak of 'teleological systems'? Analysis shows that this combination leads to definitional difficulties. 

The Greek word telos means end or goal. Teleological means end-directed. To apply the word teleological to a goal-directed behavior or process would seem quite legitimate. I am perhaps a purist, but it bothers me to apply the word teleological, that is end-directed, to a stationary system. Any phenomenon (discussed in Section 2), to which we can refer as teleomatic or teleonomic, represents a movement, a behavior, or a process, that is goal-directed by having a determinable end. This is the core concept of teleological, the presence of a 'telos' (an end) toward which an object or process moves. Rosenblueth et al. (1943) have correctly stressed the same point. 

Extending the term teleological to cover also static systems leads to contradictions and illogicalities. A torpedo that has been shot off and moves toward its target is a machine showing teleonomic behavior. But what justifies calling a torpedo a teleological system when, with hundreds of others, it is stored in an ordnance depot? Why should the eye of a sleeping person be called a teleological system? It is not goal-directed at anything. Part of the confusion is due to the fact that the term 'teleological system' has been applied to two only partially overlapping phenomena. One comprises systems that are potentially able to perform teleonomic actions, like a torpedo. The other comprises systems that are well adapted, like the eye. To refer to a phenomenon in this second class as 'teleological', in order to express its adaptive perfection, reflects just enough of the old idea of evolution leading to a steady progression in adaptation and perfection, to make me uneasy. What is telos toward which the teleological system moves?

The source of the conflict seems to be that 'goal-directed', in a more or less straightforward literal sense, is not necessarily the same as purposive. Completely stationary systems can be functional or purposive, but they cannot be goal-directed in any literal sense. A poison on the shelf has the potential of killing somebody, but this inherent property does not make it a goal-directed object. Perhaps this difficulty can be resolved by making a terminological distinction between functional properties of systems and strict goal-directedness, that is teleonomy of behavioral or other processes. However, since one will be using so-called teleological language in both cases, one might subsume both categories under teleology. 

R. Munson (1971) has recently dealt with such adaptive systems. In particular, he studied all those explanations, that deal with aspects of adaptation, but are often called 'teleological'. He designates sentences 'adaptational sentences', when they contain the terms 'adaptation', 'adaptive', or 'adapted'. In agreement with the majority opinion of biologists he concludes that "adaptational sentences do not need to involve reference to any purpose, final cause, or other non-empirical notion in order to be meaningful". Adaptational sentences simply express the conclusion that a given trait, whether structural, physiological, or behavioral, is the product of the process of natural selection and, thus favors the perpetuation of the genotype responsible for this trait. Furthermore, adaptation is a heuristic concept because it demands an answer to the question, in what way the trait adds to the probability of survival and does so more successfully than alternate conceivable trait. To me, it is misleading to call adaptational statements teleological. 'Adapted' is an a posteriori statement and it is only the success (statistically speaking), of the owner of an adaptive trait, which proves whether the trait is truly adaptive (= contributes to survival) or is not. Munson summarizes the utility of adaptational language in the sentence: "To show that a trait is adaptive is to present a phenomenon requiring explanation, and to provide the explanation is to display the success of the trait as the outcome of selection" (p. 214). The biologist fully agrees with this conclusion. Adaptive means simply: being the result of natural selection.

Many adaptive systems, as for instance all components of the locomotory and of the central nervous systems, are capable of taking part in teleonomic processes or teleonomic behavior. However, it only obscures the issue when one designates a system 'teleological' or ' teleonomic', because it provides executive structures of a teleonomic process. Is an inactive, not-programmed computer a teleological system? What 'goal' or 'end' is it displaying during this period of inactivity? To repeat, one runs into serious logical difficulties when one applies the term 'teleological' to static systems (regardless of their potential) instead of to processes. Nothing is lost and much to be gained by not using the term teleological too freely and for too many rather diverse phenomena. 

It may be necessary to coin a new term for systems which have the potential of displaying teleonomic behavior. The problem is particularly acute for biological organs which are capable of carrying out useful functions, such as pumping by the heart or filtration by the kidneys. To some extent this problem exists for any organic structure, all the way down to the macromolecules which are capable of carrying out autonomously certain highly specific functions owing to their uniquely specific structure. It is this which induced Monod (1971) to call them teleonomic systems. Similar considerations have induced some authors, erroneously in my opinion, to designate a hammer as a teleological system, because it is designed to hit a nail (a rock, not having been so designed, but serving the same function not qualifying!)

The philosophical complexity of the logical definition of 'teleological' in living systems is obvious. Let me consider a few of the proximate and ultimate causes (Mayr, 1961), to bring out some of the difficulties more clearly. The functioning of these systems is the subject matter of regulatory biology, which analyzes proximate causes. Biological systems are complicated steady state systems, replete with feedback devises. There is a high premium on homeostasis, on the maintenance of the milieu interieur. Since most of the processes performed by these systems are programmed, it is legitimate to call them teleonomic processes. They are 'end-directed' even though very often the 'end' is the maintenance of the status quo. There is nothing metaphysical in any of this because, so far as these processes are accessible to analysis, they represent chains of causally inter-related stimuli and reactions, of inputs and outputs. 

The ultimate causes for the efficiency and seeming purposefulness of these living systems were explained by Darwin in 1859. The adaptiveness of these systems is the result of millions of generations of natural selection. This is the mechanistic explanation of adaptiveness, as was clearly stated by Sigwart (1881). 

Proximate and ultimate causes must be carefully separated in the discussion of teleological systems (Mayr, 1961). A system is capable of performing teleonomic processes because it was programmed to function in this manner. The origin of the program that is responsible for the adaptiveness of the system is an entirely independent matter. It obscures definitions to combine current functioning and history of origin in a single explanation. 

5. THE HEURISTIC NATURE OF TELEONOMIC LANGUAGE

Teleological language has been employed in the past in many different senses, some of them legitimate and some of them not. When the distinctions are made, that are outline in my survey above, the teleological Fragestellung is a most powerful too in biological analysis. Its heuristic value was appreciated already by Aristotle and Galen, but neither of them fully understood why this approach is so important. Questions which being with 'What?' and 'How?' are sufficient for explanation in the physical sciences. In the biological sciences no explanation is complete until a third kind of question has been asked: 'Why?'. It is Darwin's evolutionary theory which necessitates this question: No feature (or behavioral program) of an organism ordinarily evolves unless this is favored by natural selection. It must play a role in the survival or in the reproductive success of its bearer. Accepting this premise, it is necessary for the completion of causal analysis to ask for any feature, why it exists, that is what its function and role in the life of the particular organism is. 

The philosopher Sigwart (1881) recognized this clearly:

•	A teleological analysis implies the demand to follow up causations in all directions by which the purpose [of a structure or behavior] is effected. It represents a heuristic principle because when one assumes that each organism is well adapted it requires that we ask about the operation of each individual part and that we determine the meaning of its form, its structure, and its chemical characteristics. At the same time it leads to an explanation of correlated subsidiary consequences which are not necessarily part of the same purpose but which are inevitable by-products of the same goal-directed process.

The method of course, was used successfully long before Darwin. It was Harvey's question concerning the reason for the existence of valves in the veins that made a major, if not the most important, contribution to his model of the circulation of blood. The observation that during mitosis the chromatic material is arranged in a single linear thread led Roux (1883) to question why such an elaborate process had evolved rather than a simple division of the nucleus into two halves. He concluded that the elaborate process made sense only if the chromatin consisted of an enormous number of qualitatively different small particles and that their equal division could be guaranteed only by lining them up linearly. The genetic analysis of chromosomal inheritance during the next sixty years were, in a sense, only footnotes to Roux's brilliant hypothesis. These cases demonstrate most convincingly the enormous heuristic value of the teleonomic approach. It is no exaggeration to claim that most of the greatest advances in biology were made possible by asking 'Why?' questions. This demands asking for the selective significance of every aspect of the phenotype. The former idea that many if not most characters of organisms are neutral, that is that they evolved simply as accidents of evolution, has been refuted again and again by more detailed analysis. It is the question as to the 'why?' of such structures and behaviors which initiates such analysis. Students of behavior have used this approach in recent years with great success. It has for example led to questions concerning the information content of individual vocal and visual displays (Smith, 1970; Hind, 1972).

As soon as one accepts the simple conclusion that the totality of the genotype is the result of past selection, and that the phenotype is a product of the genotype (except for the open portions of the program that are filled in during the lifetime of the individual), it becomes one's task to ask about any and every component of the phenotype what its particular functions and selective advantages are.

It is now quite evident why all past efforts to translate teleonomic statements into purely causal ones were such a failure: A crucial portion of the message of a teleological sentence is invariably lost by the translation. Let us take, for instance the sentence: 'The Wood Thrush migrates in the fall into warmer countries in order to escape the inclemency of the weather and the food shortages of the northern climates'. If we replace the words 'in order to' by 'and thereby', we leave the important question unanswered as to why the Wood Thrush migrates. The teleonomic form of the statement implies that the goal-directed migratory activity is governed by a program. By omitting this important message the translated sentence is greatly impoverished as far as information content is concerned, without gaining in causal strength. The majority of modern philosophers are fully aware of this and agree that 'cleaned-up' sentences are not equivalent to the teleological sentences from which they were derived (Ayala, 1970; Becker, 1969).

One can go one step further: Teleonomic statements have often been maligned as stultifying and obscurantist. This is simply not true. Actually the non-teleological translation is invariably a meaningless platitude while it is the teleonomic statement which leads to biologically interesting inquiries.

6. ARISTOTLE AND  TELEOLOGY

No other ancient philosopher has been as badly misunderstood and mishandled by posterity as Aristotle. His interests were primarily those of a biologist and his philosophy is bound to be misunderstood of this fact is ignored. Neither Aritotle nor most of the other ancient philosophers made a sharp distinction between the living world and the inanimate. They saw something like life or soul even in the inorganic world. If one can discern purposiveness and goal direction in the world of organisms, why not consider the order of the Kosmos-as-a-whole also as due to final causes, i.e. as due to built-in teleology? As Ayala (1970) said quite rightly, Aristotle's "error was not that he used teleological explanations in biology, but that he extended the concept of teleology to the non-living world." Unfortunately, it was this latter teleology which was first encountered during the scientific revolution of the 16th and 17th centuries (and at that in the badly distorted interpretations of the scholastics). This is one of the reasons for the violent rejection of Aristotle by Bacon, Descartes and their followers. 

Although the philosophers of the last forty years acknowledge quite generally the inspiration which Aristotle derived from the study of living nature, they still express his philosophy in words taken from the vocabulary of Greek dictionaries that are hundreds of years old. The time would seem to have come for the translators and interpreters of Aristotle to use a language appropriate to his thinking, that is the language of biology, and not that of the 16th century humanists. Delbrück (1971) is entirely right when insisting that it is quite legitimate to employ modern terms like 'genetic program' for eidos where this helps to elucidate Aristotle's thoughts. One of the reasons why Aristotle has been so consistently misunderstood is that he uses the term eidos for his form-giving principle, and everybody took it for granted that he had something in mind similar to Plato's concept of eidos. Yet, the context of Aristotle's discussions makes it abundantly clear that his eidos is something totally different from Plato's eidos (I myself did not understand this until recently). Aristotle saw with extraordinary clarity that it made no more sense to describe living organisms in terms of mere matter than to describe a house as a pile of bricks and mortar. Just as the blueprint used by the builder determines the form of a house, so does the eidos (in its Aristotelian definition) give the form to the developing organism and this eidos reflects the terminal eidos of the full grown individual. There are numerous discussions in many of Aristotle's works reflecting the same ideas. They can be found in the Analytika and in the Physics (Book II), but particularly in the Parts of Animals and in the Generation of Animals. Much of Aristotle's discussion becomes remarkably modern if one inserts modern terms to replace obsolete 16th and 17th century vocabulary. There is, of course, one major difference between Aristotle's interpretation and the modern one. Aristotle could not actually see the form-giving principle (which, after all, was not fully understood until 1953) and assumed therefore that it had to be something immaterial. When he said "Now it may be that the Form (eidos) of any living creature is soul, or some part of soul, or something that involves soul" (P. A. 641a 18), it must be remembered that Aristotle's psyche (soul) was something quite different from the conception of soul later developed in Christianity. Indeed the properties of 'soul' were to Aristotle something subject to investigation. Since the modern scientist does not actually 'see' the genetic program of DNA either, it is for him just as invisible for all practical purposes as it was for Aristotle. Its existence is inferred, as it was by Aristotle. 

As Delbrück (1971) points out correctly, Aristotle's principle of the eidos being an 'unmoved mover' is one of the greatest conceptual innovations. The physicists were particularly opposed to the existence of such principle by

•	Having been blinded for 300 years by the Newtonian view of the world. So much so, that anybody who held that the mover had to be in contact with the moved and talked about an 'unmoved mover', collided head on with Newton's dictum: action equals reaction. Any statement in conflict with this axiom of Newtonian dynamics could only appear to be muddled nonsense, a leftover from a benighted prescientific past. And yet, 'unmoved mover' perfectly describes DNA: it acts, creates form and development, and is not changed in the process (Delbrück, 1971: 55).

As I stated above, the existence of teleonomic programs – unmoved movers – is one of the most profound differences between the living and the inanimate world, and it is Aristotle who first postulated such a causation. 

7. KANT AND  TELEOLOGY

the denial of conspicuous purposiveness in living organisms and the ascription of their adaptive properties to the blind accidental interplay of forces and matter, so popular in the 18th century, was not palatable to discerning philosophyers. No one felt this more keenly than Immanuel Kant, who devoted the second half of his Critique of Judgement to the problem of teleology. It is rather surprising how completely most students of Kant ignore this work, as if they were embarrassed that the great Kant had devoted so much attention to such a 'soft' subject. Yet, as in so many other cases, Kant was more perceptive than his critics. He clearly saw two points, first that no explanation of nature is complete that cannot account for the seeming purposiveness of much of the development and behavior of living organisms, and secondly that the purely mechanical explanations available at his time were quite insufficient to explain teleological phenomena. Unfortunately, he subscribed to the prevailing dogma of his period that the only legitimate explanations were purely mechanical ('Newtonian') ones, which left him without any explanation for all teleological phenomena. He therefore concluded that the true explanation was out of our reach and that the most practical approach to the study of organisms was to deal with them 'as if they were designed'. Even though he was unable to free himself from the design-designed analogy, he stressed the heuristic value of such an approach: It permits us to make products and processes of nature far more intelligible than trying to express them purely in terms of mechanical laws. 

Kant's interest was clearly more in the explanation of 'design' (adaptation) than in teleonomic behavior, yet he thought that an explanation of design was beyond the reach of human intellect. Just 69 years before the publication of the Origin of Species Kant (1790) wrote as follows:

•	It is quite certain that we can never get a sufficient knowledge of organized beings and their inner possibility, much less explain them, according to mere mechanical principles of nature. So certain is it, that we may confidently assert that it is absurd for men to make any such attempt, or to hope that maybe another Newton will some day arrive to make intelligible to us even the production of a blade of grass according to natural laws which no design has ordered. Such insight we must absolutely deny to mankind. (quoted from McFarland, 1970).

Darwin removed the roadblock of design, and modern genetics introduced the concept of the genetic program. Between these two major advances the problem of teleology has now acquired an entirely new face. A comparison of Kan't discussion with our new concepts provides a most informative insight into the role of scientific advances in the formulation of philosophical problems. Equally informative is a comparison of three treatments of Kant's teleology, roughly separated by 50-year intervals (Stadler, 1874; Ungerer, 1922; and McFarland, 1970).

8. CONCLUSIONS

(1) The use of so-called teleological language by biologists is legitimate, it neither implies a rejection of physico-chemical explanation nor does it imply non-causal explanation,
(2) The terms teleology and teleological have been applied to highly diverse phenomena. An attempt is made by the author to group these into more or less homogeneous classes.
(3) It is illegitimate to describe evolutionary processes or trends as goal-directed (teleological). Selection rewards past phenomena (mutation, recombination, etc.), but does not plan for the future, at least not in any specific way. 
(4) Processes (behavior) whose goal-directedness is controlled by a program may be referred to as teleonomic.
(5) Processes which reach an end state caused by natural laws (e.g. gravity, second law of thermodynamics) but not by a program may be designated as teleomatic.
(6) Programs are in part or entirely the product of natural selection.
(7) The question of the legitimacy of applying the term teleological to stationary functional or adaptive systems requires further analysis.
(8) Teleonomic (i.e. programmed) behavior occurs only in organisms (and man-made machines) and constitutes a clear-cut difference between the levels of complexity in living and in inanimate nature.
(9) Teleonomic explanations are strictly causal and mechanistic. They give no comfort to adherents of vitalistic concepts.
(10) The heuristic value of the teleological Fragestellung makes it a powerful tool in biological analysis, form the study of the structural configuration of macromolecules up to the study of cooperative behavior in social systems.

ACKNOWLEDGEMENTS

Earlier versions of this manuscript were read critically by Michael Ghiselin, Stephen Gould, R. A. Hinde, Peter Medawar, Ronald Munson, W. V. Quine, Perry Turner, and William C. Wimsatt, to all of whom I am deeply indebted for numerous criticisms and suggestions, by no means all of which I adopted. I am fully aware of many remaining shortcomings, but I hope that the eventual solution of the difficult problem of teleology has been helped by my analysis. 

Postcript: A number of discussions of teleology have come to my attention since the manuscript was completed. R. Jakobson (1970) presents a perceptive analysis of the indispensable uses (as well as limitations) of teleological language. He stresses the remarkable similarity between language and the genetic program in their ability to combine simple elements ('letters') into 'words' and 'sentences' which convey information efficiently. William D. Wimsatt (1971) deals with functional systems which he analyzes in terms of selection processes. Although I entirely agree with his analysis, it would seem to me that he merely confirms that the term teleological is superfluous when applied to systems. However, his analysis (as does that of Munson) raises the problem of how to delimit functional against teleological language. 

NOTE

1. This is quite evident from the following explanatory comment I have received from Professor Pittendrigh by letter (dated February 26, 1970):

You ask about the word 'teleonomy'. You are correct that I did introduce the term into biology and, moreover, I invented it. In the course of thinking about that paper which I wrote for the Simpson and Roe book (in which the term is introduced) I was haunted by that famous old quip of Haldane's to the effect that 'Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public'. The more I thought about that, it occurred to me that the whole thing was nonsense - that what it was the biologist couldn't live with was not the illegitimacy of the relationship, but the relationship itself. Teleology in the Aristotelian form has, of course, the end as immediate, 'efficient' cause. And that is precisely what the biologist (with the whole history of science since 1500 behind him) cannot accept: it is unacceptable in a world that is always mechanistic (and of course in this I include probabilistic as well as strictly deterministic). What it was the biologist could not escape was the plain fact – or rather the fundamental fact – which he must (as scientist) explain: that the objects of biological analysis are organizations (he calls them organisms) and, as such, are end-directed. Organization is more that mere order; order lacks end-directedness; organization is end-directed. [I recall a wonderful conversation with John von Neumann in which we explored the difference between 'mere order' and 'organization' and his insistence (I already believed it) that the concept of organization (as contextually defined in its everyday use) always involved 'purpose' or end-directedness.

I wanted a word that would allow me (all of us biologists) to describe, stress or simply to allude to – without offense – this end-directedness of a perfectly respectable mechanistic system. Teleology would not do, carrying with it that implication that the end is causally effective in the current operation of the machine. Teleonomic, it is hoped, escapes that plain falsity which is anyhow unnecessary. Haldane was, in this sense wrong (surely a rare event): we can live without teleology. 

The crux of the problem lies of course in unconfounding the mechanism of evolutionary change and the physiological mechanism of the organism abstracted from the evolutionary time scale. The most general of all biological 'ends', or 'purposes' is of course perpetuation by reproduction. That end [and all its subsidiary 'ends' of feeding, defense and survival generally] is in some sense effective in causing natural selection; in causing evolutionary change; but not in causing itself. In brief, we have failed in the past to unconfound causation in the historical origins of a system and causation in the contemporary working of the system…

You ask in your letter whether or not one of the 'information' people didn't introduce it. They did not, unless you wish to call me an information bloke. It is, however, true that my own thinking about the whole thing was very significantly affected by a paper which was published by Wiener and Bigelow with the intriguing title 'Purposeful machines'. This pointed out that in the then newly-emerging computer period it was possible to design and build machines that had ends or purposes without implying that the purposes were the cause of the immediate operation of the machine. 

BIBLIOGRAPHY

Ayala, F. J., 'Teleological Explanations in Evolutionary Biology', Phil. Sci. 37 (1970) 1-15.

Baer, K. E. von, 'Uber den Zweck in den Vorgängen der Natur'. Studien, etc. (1876) 49-105, 170-234, St. Petersburg.

Beckner, M., 'Function and teleology', J. Hist. Biol 2 (1969) 151-164.

Bergson, H., Evolution Créative, Alcan, Paris, 1907.

Braithwaite, R. D., Scientific Explanation, Cambridge Univ. Press, 1954 (also in Canfield, pp 27-47). 

Canfield, J. V., (ed.) Purpose in Nature. Prentice-hall, Englewood Cliffs, N. J. 1966, p. 1-7.

Craig, W., 'Appetites and Aversions as Constituents of Instincts', Bio. Bull. 34 (1918) 91-107.

Davis, B. D., 'The Teleonomic Significance of Biosynthetic Control Mechanisms'. Cold Spring Harbor Symposia 26 (1961), 1-10.

Delbrück, M., 'Aristotle-totle-totle', in Of Microbes and Life (ed. by J. Monod and E. Borek), Columbia University Press, New York, 1971.

Driesch, H., Philosophie des Organischen, Quelle und Meyer, Leipzig, 1909. 

Hinde, R. A., (ed.), Non-Verbal Communication, Cambridge University Press, 1972.

Hinde, R. A. and Stevenson, J. G., 'Goals and Response Controls', in Development and Evolution of Behavior (ed. by L. R. Aronson et al), Freeman, 1970.

Hull, D., Philosophy of Biological Science, Foundations of Philosophy Series, Prentice-Hall, Englewood Cliffs, N. J., 1973.

Jakobsen, R., 'Linguistics', in Main Trends of Research in the Social and Human Sciences, Chapter 6, 1970.

Kant, I., Kritik der Urteilskraft, Zweiter Teil, 1790.

Lagerspetz, K., 'Teleological Explanations and Terms in Biology', Ann Zool. Soc. Venamo 19 (1959) 1-73.

Lehman, H., "Functional Explanations in Biology', Philos. Sci. 32 (1965) 1-20.

Lovejoy, A. O., The Great Chain of Being, Harvard University Press, Cambridge, Mass., 1936.

MacLeod, R. T. 'Teleology and Theory of Human Behavior', Science 125 (1957) 477.

Mains, F., Foundations of Biology, Foundations of the Unity of Science, I (9) (1955) 1-86.

Mayr, E., 'Cause and Effect in Biology', Science 134 (1961) 1501-1506.

Mayr, E., 'The Evolution of Living Systems', Proc. Nat. Acad. Sci. 51 (1964) 934-941.

McFarland, J. D., Kant's Concept of Teleology, Univ. of Edinburg Press, 1970. 

Monod, J., Chance and Necessity, Alfred A. Knopf, New York, 1971.

Munson, R., 'Biological Adaptation', Phil. Sci. 38 (1971) 200-215.

Nagel, E., 'The Structure of Teleological Explanations', in The Structure of Science, Harcourt, Brace, and World, 1961.

Pittendrigh, C. S. Behavior and Evolution (ed. by A. Rose and G. G. Simpson), Yale University Press, New Haven, 1958.

Raven, Chr. P., 'The Formalization of Finality', Folia Biotheoretica V (1960) 1-27.

Roe, A. and Simpson, G. G., (eds.) Behavior and Evolution (ed. by A. Rose and G. G. Simpson), Yale University Press, New Haven, 1958.

Rosenblueth, H., Wiener, N., and Bigelow, J., 'Behavior, Purpose, and Teleology', Philos. Sci. 10 (1943) 18-24 (also in Canfield, pp. 9-16).

Roux, W., Über die Bedeutung der Kerntheilungsfiguren. Eine hypothetische Erörterung, Leipzig, 1883.

Sigwart, C., Der Kampf gegen den Zweck. Kleine Schriften 2, Mohr, Freiburg, 1881, pp 24-67.

Simpson, G. G., The Meaning of Evolution: a study of the history of life and of its significance for man. Yale University Press, New Haven, 1949. 

Smith, W. John., 'Messages of Vertebrate Communication', Science 165 (1969) 145-150.

Sommerhoff, G., Analytical Biology, Oxford University Press, London, 1950. 

Stadler, H., Kant's Teleologie und ihre erkentnistheoretische Bedeutung, F. Dümmler, 1874.

Taylor, R., 'Comments on a Mechanistic Conception of Purposefulness', Phil. Sci. 17 (1950) 310-317 (also in Canfield, pp. 17-26).

Teilhard de Chardin, P., Le Phénoméne Humain, Editions de Seuil, Paris, 1955.

Theiler, W., Zur Geschichte der Teleologischen Naturbetrachtung bis Aristoteles, Zürich und Leipzig, 1925.

Ungerer, E., Die Teleologie Kants und ihre Bedeutung für die Logik der Biologie, Bornträger, Berlin, 1922. 

Waddington, C. H., The Strategy of the Genes, George Allen & Unwin Ltd., London, 1957, pp. 11-58. 

Waddington, C. H., Towards a Theoretical Biology 1, Edinburg University Press, 1968, pp. 55-56.

Weismann, A., 'The Selection Theory', in Darwin and Modern Science (ed. by A. C. Seward), Cambridge University Press, 1909.

Wimsatt, W. C., "Teleology and the Logical Structure of Function Statements', Stud. Hist. Phil. Sci. 3 (1972) 1-80.