Aristotle (384-322 B.C.E.) may be said to be the first biologist in the Western tradition. Though there are physicians and other natural philosophers who remark on various flora and fauna before Aristotle, none of them brings to his study a systematic critical empiricism. Aristotle’s biological science is important to understand, not only because it gives us a view into the history and philosophy of science, but also because it allows us more deeply to understand his non-biological works, since certain key concepts from Aristotle’s biology repeat themselves in his other writings. Since a significant portion of the corpus of Aristotle’s work is on biology, it is natural to expect his work in biology to resonate in his other writings. One may, for example, use concepts from the biological works to better understand the ethics or metaphysics of Aristotle.
This article will begin with a brief explanation of his biological views and move toward several key explanatory concepts that Aristotle employs. These concepts are essential because they stand as candidates for a philosophy of biology. If Aristotle’s principles are insightful, then he has gone a long way towards creating the first systematic and critical system of biological thought. It is for this reason (rather than the particular observations themselves) that moderns are interested in Aristotle’s biological writings.
Table of Contents
1. His Life
Aristotle was born in the year 384 B.C. in the town of Stagira (the modern town Stavros), a coastal Macedonian town to the north of Greece. He was raised at the court of Amyntas where he probably met and was friends with Philip (later to become king and father to Alexander, the Great). When Aristotle was around 18, he was sent to Athens to study in Plato’s Academy. Aristotle spent twenty years at the Academy until Plato’s death, although Diogenes says Aristotle left before Plato’s death. When Plato was succeeded by his nephew, Speusippus, as head of the Academy, Aristotle accepted an invitation to join a former student, Hermeias, who was gathering a Platonic circle about him in Assos in Mysia (near Troy). Aristotle spent three years in this environment. During this time, he may have done some of the natural investigations that later became The History of Animals.
At the end of Aristotle’s stay in Mysia, he moved to Lesbos (an adjacent island). This move may have been prompted by Theophrastus, a fellow of the Academy who was much influenced by Aristotle. It is probable (according to D’Arcy Thompson) that Aristotle performed some important biological investigations during this period.
Aristotle returned to Athens (circa 334-5). This began a period of great productivity. He rented some grounds in woods sacred to Apollo. It was here that Aristotle set-up his school (Diog. Laert V, 51).
At his school Aristotle also accumulated a large number of manuscripts and created a library that was a model for later libraries in Alexandria and Pergamon. According to one tradition, Alexander (his former pupil) paid him a handsome sum of money each year as a form of gratitude (as well as some exotic animals for Aristotle to study that Alexander encountered in his conquests).
At the death of Alexander in 323, Athens once again was full of anti-Macedonian sentiment. A charge of impiety was brought against Aristotle due to a poem he had written for Hermeias. One martyr for philosophy (Socrates) was enough for Aristotle and so he left his school to his colleague, Theophrastus, and fled to the Macedonian Chalcis. Here in 322 he died of a disease that is still the subject of speculation.
2. The Scope of Aristotle’s Biological Works
There is some dispute as to which works should be classified as the biological works of Aristotle. This is indeed a contentious question that is especially difficult for a systematic philosopher such as Aristotle. Generally speaking, a systematic philosopher is one who constructs various philosophical distinctions that, in turn, can be applied to a number of different contexts. Thus, a distinction such as “the more and the less” that has its roots in biology explaining that certain animal parts are greater (bigger) among some individuals and smaller among others, can also be used in the ethics as a cornerstone of the doctrine of the mean as a criterion for virtue. That is, one varies from the mean by the principle of the more and the less. For example, if courage is the mean, then the defect of excess would be “foolhardiness” while the defect of paucity would be “cowardice.” The boundary between what we’d consider “biology” proper vs. what we’d think of as psychology, philosophy of mind, and metaphysics is often hard to draw in Aristotle. That’s because Aristotle’s understanding of biology informs his metaphysics and philosophy of mind, but likewise, he often uses the distinctions drawn in his metaphysics in order to deal with biological issues.
In this article, the biological works are: (a) works that deal specifically with biological topics such as: The Parts of Animals (PA), The Generation of Animals (GA), The History of Animals (HA), The Movement of Animals, The Progression of Animals, On Sense and Sensible Objects, On Memory and Recollection, On Sleep and Waking, On Dreams, On Prophecy in Sleep, On Length and Shortness of Life, On Youth and Old Age, On Life and Death, On Respiration, On Breath, and On Plants, and (b) the work that deals with psuche (soul), On the Soul— though this work deals with metaphysical issues very explicitly, as well. This list does not include works such as the Metaphysics, Physics, Posterior Analytics, Categories, Nicomachean Ethics, or The Politics even though they contain many arguments that are augmented by an understanding of Aristotle’s biological science. Nor does this article examine any of the reputedly lost works (listed by ancient authors but not existing today) such as Dissections, On Composite Animals, On Sterility, On Physiognomy, and On Medicine. Some of these titles may have sections that have survived in part within the present corpus, but this is doubtful.
3. The Specialist and the Generalist
The distinction between the specialist and the generalist is a good starting point for understanding Aristotle’s philosophy of biology. The specialist is one who has a considerable body of experience in practical fieldwork while the generalist is one who knows many different areas of study. This distinction is brought out in Book One of the Parts of Animals (PA). At PA 639a 1-7 Aristotle says,
In all study and investigation, be it exalted or mundane, there appear to be two types of proficiency: one is that of exact, scientific knowledge while the other is a generalist’s understanding. (my tr.)
Aristotle does not mean to denigrate or to exalt either. Both are necessary for natural investigations. The generalist’s understanding is holistic and puts some area of study into a proper genus, while scientific knowledge deals with causes and definitions at the level of the species. These two skills are demonstrated by the following example:
An example of what I mean is the question of whether one should take a single species and state its differentia independently, for example, homo sapiens nature or the nature of Lions or Oxen, etc. or should we first set down common attributes or a common character (PA 639a 15-19, my tr.).
In other words, the methodology of the specialist would be to observe and catalogue each separate species by itself. The generalist, on the other hand, is drawn to making more global connections through an understanding of the common character of many species. Both skills are needed. Here and elsewhere Aristotle demonstrates the limitations of a single mode of discovery. We cannot simply set out a single path toward scientific investigation—whether it be demonstrative (logical) exactness (the specialist’s understanding) or holistic understanding (the generalist’s knowledge). Neither direction (specialist or generalist) is the one and only way to truth. Really, it is a little of both working in tandem. Sometimes one half takes the lead and sometimes the other. The adoption of several methods is a cornerstone of Aristotelian pluralism, a methodological principle that characterizes much of his work.
When discussing biological science, Aristotle presents the reader two directions: (a) the modes of discovery (genetic order) and (b) the presentation of a completed science (logical order). In the mode of discovery, the specialist sets out all the phenomena in as much detail as possible while the generalist must use her inter-generic knowledge to sort out what may or may not be significant in the event taking place before her. This is because in the mode of discovery, the investigator is in the genetic order. Some possible errors that could be made in this order (for example) might be mistaking certain animal behaviors for an end for which they were not intended. For example, it is very easy to mistake mating behavior for aggressive territorial behavior. Since the generalist has seen many different types of animals, she may be in the best position (on the basis of generic analogy) to classify the sort of behavior in question.
In the mode of discovery one begins with the phenomenon and then seeks to create a causal explanation (PA 646a 25). But how does one go about doing this? In the Posterior Analytics II.19, Aristotle suggests a process of induction that begins with the particular and then moves to the universal. Arriving at the universal entails a comprehensive understanding of some phenomenon. For example, if one wanted to know whether fish sleep, one would first observe fish in their environment. If one of the behaviors of the fish meets the common understanding of sleep (such as being deadened to outside stimulus, showing little to no movement, and so forth), then one may move to the generalization that fish sleep (On Sleeping and Waking 455b 8, cf. On Dreams 458b 9). But one cannot stop there. Once one has determined that fish sleep (via the inductive mode of discovery), it is now up to the researcher to ferret out the causes and reasons why, in a systematic fashion. This second step is the mode of presentation. In this mode the practitioner of biological science seeks to understand why the universal is as it is. Going back to the example of sleeping fish, the scientist would ask why fish need to sleep. Is it by analogy to humans and other animals that seem to gather strength through sleep? What ways might sleep be dangerous (say by opening the individual fish to being eaten)? What do fish do to avoid this?
These, and other questions require the practitioner to work back and forth with what has been set down in the mode of discovery for the purpose of providing an explanation. The most important tools for this exercise are the two modes of causal explanation.
4. The Two Modes of Causal Explanation
For Aristotle there are four causes: material, efficient, formal, and final. The material cause is characterized as “That out of which something existing becomes” (Phys. 194b 24). The material has the potential for the range of final products. Within the material is, in a potential sense, that which is to be formed. Obviously, one piece of wood or metal has the potential to be many artifacts; yet the possibilities are not infinite. The material itself puts constraint upon what can be produced from it. One can execute designs in glass, for example, which could never be brought forth from brass.
The efficient cause is depicted as “that from whence comes the first principle of kinetic change or rest” (Phys. 194b 30). Aristotle gives the example of a male fathering a child as showing an efficient cause. The efficient cause is the trigger that starts a process moving.
The formal cause constitutes the essence of something while the final cause is the purpose of something. For example, Aristotle believed the tongue to be for the purpose of either talking or not. If the tongue was for the purpose of talking (final cause), then it had to be shaped in a certain way, wide and supple so that it might form subtle differences in sound (formal cause). In this way the purpose of the tongue for speaking dovetails with the structural way it might be brought about (P.A. 660a 27-32).
It is generally the case that Aristotle in his biological science interrelates the final and formal causes. For example Aristotle says that the efficient cause may be inadequate to explain change. In the On Generation and Corruption 336a Aristotle states that all natural efficient causes are regulated by formal causes. “It is clear then that fire itself acts and is acted upon.” What this means is that while the fire does act as efficient cause, the manner of this action is regulated by a formal/final cause. The formal cause (via the doctrine of natural place—that arranges an ascending hierarchy among the elements, earth, water, air and fire) dictates that fire is the highest level of the sub-lunar phenomena. Thus, its essence defines its purpose, namely, to travel upward toward its own natural place. In this way the formal and final cause act together to guide the actions of fire (efficient cause) to point upward toward its natural place.
Aristotle (at least in the biological works) invokes a strategy of redundant explanation. Taken at its simplest level, he gives four accounts of everything. However, in the actual practice, it comes about that he really only offers two accounts. In the first account he presents a case for understanding an event via material/kinetic means. For the sake of simplicity, let us call this the ME (materially-based causal explanation) account.
In the second case he presents aspects of essence (formal cause) and purpose (final cause). These are presented together. For the sake of simplicity, let us call this the TE (teleologically-based causal explanation) account. For an example of how these work together, consider respiration.
Aristotle believes that material and efficient causes can give one account of the motions of the air in and out of the lungs for respiration. But this is only part of the story. One must also consider the purpose of respiration and how this essence affects the entire organism (PA 642a 31-642b 4). Thus the combination of the efficient and material causes are lumped together as one sort of explanation ME that focus upon how the nature of hot
and cold air form a sort of current that brings in new air and exhales the old. The final and formal causes are linked together as another sort of explanation TE that is tied to why we have respiration in the first place.
In Aristotle’s account respiration we are presented with a partner to TE and ME: necessity. When necessity attaches itself to ME it is called simple or absolute necessity. When necessity attaches itself to TE it is called conditional necessity. Let us return to our example of respiration and examine these concepts in more detail.
First, then there is the formal/final cause of respiration. Respiration exists so that air might be brought into the body for the creation of pneuma (a vital force essential for life). If there were no respiration, there would be no intake of air and no way for it to be heated in the region of the heart and turned intopneuma —an element necessary for life among the blooded animals who live out of water. Thus the TE for respiration is for the sake of producing an essential raw material for the creation of pneuma.
The second mode of explanation, ME, concerns the material and efficient causes related to respiration. These have to do with the manner of a quasi-gas law theory. The hot air in the lungs will tend to stay there unless it is pushed out by the cold incoming air that hurries its exit (cf. On Breath 481b 11). (This is because ‘hot’ and ‘cold’ are two of the essential contraries hot/cold & wet/dry). It is the material natures of the elements that dictate its motions. This is the realm of the ME.
ME is an important mode of explanation because it grounds the practitioner in the empirical facts so that he may not incline himself to offer mere a priori causal accounts. When one is forced to give material and kinetic accounts of some event, then one is grounded in the tangible dynamics of what is happening. This is one important requirement for knowledge.
Now to necessity. Necessity can be represented as a modal operator that can attach itself to either TE or to ME. When it attaches itself to TE, the result is conditional necessity. In conditional necessity one must always begin with the end to be achieved. For example, if one assumes the teleological assumption of natural efficiency, Nature does nothing in vain (GA 741b 5, cf. 739b20, et. al.) then the functions of various animal parts must be viewed within that frame. If we know that respiration is necessary for life, then what animal parts are necessary to allow respiration within different species? The acceptance of the end of respiration causes the investigator to account for how it can occur within a species. The same could be said for other given ends such as “gaining nutrition,” “defending one’s self from attack,” and “reproduction,” among others. When the biologist begins his investigation with some end (whether in the mode of discovery or the mode of scientific presentation), he is creating an account of conditional necessity.
The other sort of necessity is absolute necessity that is the result of matter following its nature (such as fire moving to its natural place). The very nature of the material, itself, creates the dynamics—such as the quasi gas law interactions between the hot and cold air in the lungs. These dynamics may be described without proximate reference to the purpose of the event. In this way ME can function by itself along with simple necessity to give one complete account of an event.
In biological science Aristotle believes that conditional necessity is the most useful of the two necessities in discovery and explanation (PA 639b 25). This is because, in biology, there is a sense that the entire explanation always requires the purpose to set out the boundaries of what is and what is not significant. However, in his practice it is most often the case that Aristotle employs two complete accounts ME and TE in order to reveal different modes of explanation according to his doctrine of pluralism.
5. Aristotle’s Theory of Soul
The word for ‘soul’ in Aristotle is psuche. In Latin it is translated as anima. For many readers, it is the use of the Latin term (particularly as it was used by Christian, Moslem, and Jewish theologians) that forms the basis of our modern understanding of the word. Under the theological tradition, the soul meant an immaterial, detached ruling power within a human. It was immortal and went to God after death. This tradition gave rise to Descartes’ metaphysical dualism: the doctrine that there are two sorts of things that exist (soul and matter), and that soul ruled matter.
Aristotle does not think of soul as the aforementioned theologians do. This is because matter (hyle ) and shape (morphe ) combine to create a unity not a duality. The philosopher can intellectually abstract out the separate constituents, but in reality they are always united. This unity is often termed hylomorphism (after its root words). Using the terminology of the last section we can identify hyle with ME and morphe with TE. Thus, Aristotle’s doctrine of the soul (understood as hylomorphism) represents a unity of form and function within matter.
From the biological perspective, soul demarcates three sorts of living things: plants, animals, and human beings. In this way soul acts as the cause of a body’s being alive (De An 415b 8). This amalgamation (soul and body) exhibits itself through the presentation of a particular power that characterizes what it means to be alive for that sort of living thing.
The soul is the form of a living body thus constituting its first actuality. Together the body and soul form an amalgamation. This is because when we analyze the whole into its component parts the particular power of the amalgamation is lost. Matter without TE, as we have seen, acts through the nature of its elements (earth, air, fire, and water) and not for its organic purpose. An example that illustrates the relationship between form and matter is the human eye. When an eye is situated in a living body, the matter (and the motions of that matter) of the eye works with the other parts of the body to present the actualization of a particular power: sight. When governed by the actuality (or fulfillment) of its purpose, an eyeball can see (De An 412b 17). Both the matter of the eyeball and its various neural connections (hyle, understood as ME) along with the formal and final causes (morphe, understood as TE) are necessary for sight. Each part has its particular purpose, and that purpose is given through its contribution to the basic tasks associated with essence of the sort of thing in question: plant, animal, human.
It is important not to slip into the theological cum Cartesian sense of anima here. To say that plants and animals have souls is not to assert that there is a Divine rose garden or hound Heaven. We must remember that soul for Aristotle is a hylomorphic unity representing a monism and not a dualism. (The rational soul’s status is less clear since it is situated in no particular organ since Aristotle rejected the brain as the organ of thinking relegating it to a cooling mechanism, PA652b 21-25). It is the dynamic, vital organizing principle of life—nothing more, nothing less.
Plants exhibit the most basic power that living organisms possess: nutrition and reproduction (De An 414a 31). The purpose of a plant is to take in and process materials in such a way that the plant grows. Several consequences follow (for the most part) from an individual plant having a well-operating nutritive soul. Let’s examine one sort of plant, a tree. If a plant exhibits excellence in taking in and processing nutrition it will exhibit various positive effects. First, the tree will have tallness and girth that will see it through different weather conditions. Second, it will live longer. Third, it will drop lots of seeds giving rise to other trees. Thus, if we were to compare two individual trees (of the same species), and one was tall and robust while the other was small and thin, then we would be able to render a judgment about the two individual trees on the basis of their fulfillment of their purpose as plants within that species. The tall and robust tree of that species would be a better tree (functionally). The small and thin tree would be condemned as failing to fulfill its purpose as a plant within that species.
Animals contain the nutritive soul plus some of the following powers: appetite, sensation, and locomotion (De An 414a 30, 414b 1-415a 13). Now, not all animals have all the same powers. For example, some (like dogs) have a developed sense of smell, while others (like cats) have a developed ability to run quickly with balance. This makes simple comparisons between species more difficult, but within one species the same sort of analysis used with plants also holds. That is, between two individual dogs one dog can (for example) smell his prey up to 200 meters away while the other dog can only detect his prey up to 50 meters. (This assumes that being able to detect prey from a distance allows the individual to eat more often.) The first dog is better because he has fulfilled his soul’s function better than the second. The first dog is thus a good dog while the second a bad example of one. What is important here is that animals judged as animals must fulfill that power (soul) particular to it specifically in order to be functionally excellent. This means that dogs (for example) are proximately judged on their olfactory sense and remotely upon their ability to take in nutrition and to reproduce.
Humans contain the nutritive soul and the appetitive-sensory-locomotive souls along with the rational soul. This power is given in a passive, active, and imaginative sense (De An III 3-5). What this means is that first there is a power in the rational soul to perceive sensation and to process it in such a way that it is intelligible. Next, one is able to use the data received in the first step as material for analysis and reflection. This involves the active agency of the mind. Finally, the result (having both a sensory and ratiocinative element) can be arranged in a novel fashion so that the universal mixes with the perceived particular. This is imagination (De An III.3). For example, one might perceive in step-one that your door is hanging at a slant. In step-two you examine the hinges and ponder why the door is hanging in just this way. Finally, in step-three you consider types of solutions that might solve the problem—such as taking a plane to the top of the door, or inserting a “shim” behind one of the hinges. You make your decision about this door in front of you based upon your assessment of the various generic solutions.
The rational soul, thus understood as a multi-step imaginative process, gives rise to theoretical and practical knowledge that, in turn, have other sub-divisions (EN VI). Just as the single nutritive soul of plants was greatly complicated by the addition of souls for the animals, so also is the situation even more complicated with the addition of the rational soul for humans. This is because it has so many different applications. For example, one person may know right and wrong and can act on this knowledge and create habits of the same while another may have productive knowledge of an artist who is able to master the functional requirements of his craft in order to produce well-wrought artifacts. Just as it is hard to compare cats and dogs among animal souls, so it is difficult to judge various instantiations of excellence among human rational souls. However, it is clear that between two persons compared on their ethical virtues and two artists compared on their productive wisdom, we may make intra-category judgments about each. These sorts of judgments begin with a biological understanding of what it means to be a human being and how one may fulfill her biological function based on her possession of the human rational soul (understood in one of the sub-categories of reason). Again, a biological understanding of the soul has implications beyond the field of biology/psychology.
6. The Biological Practice: Outlines of a Systematics
Systematics is the study of how one ought to create a system of biological classification and thus perform taxonomy. (“Systematics” is not to be confused with being a “systematic philosopher.” The former term has a technical meaning related to the theoretical foundations of animal classification and taxonomy. The latter phrase has to do with a tightly structured interlocking philosophical account.) In Aristotle’s logical works, he creates a theory of definition. According to Aristotle, the best way to create a definition is to find the proximate group in which the type of thing resides. For example, humans are a type of thing (species ) and their proximate group is animal (or blooded animal). The proximate group is called thegenus. Thus the genus is a larger group of which the species is merely one proper subset. What marks off that particular species as unique? This is the differentia or the essential defining trait. In our example with humans the differentia is “rationality.” Thus the definition of “human” is a rational animal. “Human” is the species, “animal” is the genus and “rationality” is the differentia.
In a similar way, Aristotle adapts his logical theory of genus and species to biology. By thinking in terms of species and their proximate genus, Aristotle makes a statement about the connections between various types of animals. Aristotle does not create a full-blown classification system that can describe all animals, but he does lay the theoretical foundations for such.
The first overarching categories are the blooded and the non-blooded animals. The animals covered by this distinction roughly correspond to the modern distinction between vertebrates and invertebrates. There are also two classes of dualizers that are animals that fit somewhat between categories. Here is a sketch of the categorization:
I. Blooded Animals
A. Live bearing animals
1. Homo Sapiens2. Other mammals without a distinction for primates