Aristotle's biology is the theory of biology, grounded in systematic observation and collection of data, mainly zoological, embodied in Aristotle's books on the science. Many of his observations were made during his stay on the island of Lesbos, including especially his descriptions of the marine biology of the Pyrrha lagoon, now the Gulf of Kalloni. His theory is based on his concept of form, which derives from but is markedly unlike Plato's theory of Forms.
The theory describes five major biological processes, namely metabolism, temperature regulation, information processing, embryogenesis, and inheritance. Each was defined in detail, in some cases enough allowing modern biologists to construct mathematical models of the mechanisms described. Aristotle's method, too, resembled the style of science used by modern biologists when exploring a new area, with systematic data collection, discovery of patterns, and inference of possible causal explanations from these. He did not perform experiments in the modern sense, but made observations of living animals and carried out dissections. He names some 500 species of bird, mammal, and fish; and he distinguishes dozens of insects and other invertebrates. He describes the internal anatomy of over a hundred animals, and dissected around 35 of these.
Aristotle's writings on biology, the first in the history of science, are scattered across several books, forming about a quarter of his writings that have survived. The main biology texts were the History of Animals, Generation of Animals, Movement of Animals, Progression of Animals, Parts of Animals, and On the Soul, as well as the lost drawings of The Anatomies which accompanied the History.
Apart from his pupil, Theophrastus, who wrote a matching Enquiry into Plants, no research of comparable scope was carried out in ancient Greece, though Hellenistic medicine in Egypt continued Aristotle's inquiry into the mechanisms of the human body. Aristotle's biology was influential in the medieval Islamic world. Translation of Arabic versions and commentaries into Latin brought knowledge of Aristotle back into Western Europe, but the only biological work widely taught in medieval universities was On the Soul. The association of his work with medieval scholasticism, as well as errors in his theories, caused Early Modern scientists such as Galileo and William Harvey to reject Aristotle. Criticism of his errors and secondhand reports continued for centuries. He has found better acceptance among zoologists, and some of his long-derided observations in marine biology have been found in modern times to be true.
Context
Aristotle's background
Aristotle (384–322 BC) studied at Plato's Academy in Athens, remaining there for about 20 years. Like Plato, he sought universals in his philosophy, but unlike Plato he backed up his views with detailed and systematic observation, notably of the natural history of the island of Lesbos, where he spent about two years, and the marine life in the seas around it, especially of the Pyrrha lagoon in the island's centre. This study makes him the earliest scientist whose written work survives. No similarly detailed work on zoology was attempted until the sixteenth century; accordingly Aristotle remained highly influential for some two thousand years. He returned to Athens and founded his own school, the Lycaeum, where he taught for the last dozen years of his life. His writings on zoology form about a quarter of his surviving work. Aristotle's pupil Theophrastus later wrote a similar book on botany, Enquiry into Plants.
Aristotelian forms
Aristotle's biology is constructed on the basis of his theory of form, which is derived from Plato's theory of Forms, but significantly different from it. Plato's Forms were eternal and fixed, being "blueprints in the mind of God". Real things in the world could, in Plato's view, at best be approximations to these perfect Forms. Aristotle heard Plato's view and developed it into a set of three biological concepts. He uses the same Greek word, (eidos), to mean first of all the set of visible features that uniquely characterised a kind of animal. Aristotle used the word γένος (génos) to mean a kind. For example, the kind of animal called a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood.
Aristotle further noted that there are many bird forms within the bird kind – cranes, eagles, crows, bustards, sparrows, and so on, just as there are many forms of fishes within the fish kind. He sometimes called these atoma eidē, indivisible forms. Human is one of these indivisible forms: Socrates and the rest of us are all different individually, but we all have human form. More recent studies have shown that Aristotle used the terms γένος (génos) and (eidos) in a relative way. A taxon that is considered an eidos in one context can be considered a génos (which includes various eide) in another.
Finally, Aristotle observed that the child does not take just any form, but is given it by the parents' seeds, which combine. These seeds thus contain form, or in modern terms information. Aristotle makes clear that he sometimes intends this third sense by giving the analogy of a woodcarving. It takes its form from wood (its material cause); the tools and carving technique used to make it (its efficient cause); and the design laid out for it (its eidos or embedded information). Aristotle further emphasises the informational nature of form by arguing that a body is compounded of elements like earth and fire, just as a word is compounded of letters in a specific order.
System
Soul as system
As analysed by the evolutionary biologist Armand Leroi, Aristotle's biology included five major interlocking processes:
a metabolic process, whereby animals take in matter, change its qualities, and distribute these to use to grow, live, and reproduce a cycle of temperature regulation, whereby animals maintain a steady state, but which progressively fails in old age an information processing model whereby animals receive sensory information, alter it in the seat of sensation, and use it to drive movements of the limbs. He thus separated sensation from thought, unlike all previous philosophers except Alcmaeon. the process of inheritance. the processes of embryonic development and of spontaneous generation
The five processes formed what Aristotle called the soul: it was not something extra, but the system consisting exactly of these mechanisms. The Aristotelian soul died with the animal and was thus purely biological. Different types of organism possessed different types of soul. Plants had a vegetative soul, responsible for reproduction and growth. Animals had both a vegetative and a sensitive soul, responsible for mobility and sensation. Humans, uniquely, had a vegetative, a sensitive, and a rational soul, capable of thought and reflection.Aristotle, De Anima II 3
Processes
Metabolism
Aristotle's account of metabolism sought to explain how food was processed by the body to provide both heat and the materials for the body's construction and maintenance. The metabolic system for live-bearing tetrapods described in the Parts of Animals can be modelled as an open system, a branching tree of flows of material through the body.
The system worked as follows. The incoming material, food, enters the body and is concocted into blood; waste is excreted as urine, bile, and faeces, and the element fire is released as heat. Blood is made into flesh, the rest forming other earthy tissues such as bones, teeth, cartilages and sinews. Leftover blood is made into fat, whether soft suet or hard lard. Some fat from all around the body is made into semen.
All the tissues are in Aristotle's view completely uniform parts with no internal structure of any kind; a cartilage for example was the same all the way through, not subdivided into atoms as Democritus (c. 460–c. 370 BC) had argued. The uniform parts can be arranged on a scale of Aristotelian qualities, from the coldest and driest, such as hair, to the hottest and wettest, such as milk.
At each stage of metabolism, residual materials are excreted as faeces, urine, and bile.
Temperature regulation
Aristotle's account of temperature regulation sought to explain how an animal maintained a steady temperature and the continued oscillation of the thorax needed for breathing. The system of regulation of temperature and breathing described in Youth and Old Age, Life and Death 26 is sufficiently detailed to permit modelling as a negative feedback control system (one that maintains a desired property by opposing disturbances to it), with a few assumptions such as a desired temperature to compare the actual temperature against.
The system worked as follows. Heat is constantly lost from the body. Food products reach the heart and are processed into new blood, releasing fire during metabolism, which raises the blood temperature too high. That raises the heart temperature, causing lung volume to increase, in turn raising the airflow at the mouth. The cool air brought in through the mouth reduces the heart temperature, so the lung volume accordingly decreases, restoring the temperature to normal.
The mechanism only works if the air is cooler than the reference temperature. If the air is hotter than that, the system becomes a positive feedback cycle, the body's fire is put out, and death follows. The system as described damps out fluctuations in temperature. Aristotle however predicted that his system would cause lung oscillation (breathing), which is possible given extra assumptions such as of delays or non-linear responses.
Information processing
Aristotle's information processing model has been named the "centralized incoming and outgoing motions model". It sought to explain how changes in the world led to appropriate behaviour in the animal.
The system worked as follows. The animal's sense organ is altered when it detects an object. This causes a perceptual change in the animal's seat of sensation, which Aristotle believed was the heart (cardiocentrism) rather than the brain. This in turn causes a change in the heart's heat, which causes a quantitative change sufficient to make the heart transmit a mechanical impulse to a limb, which moves, moving the animal's body. The alteration in the heat of the heart also causes a change in the consistency of the joints, which helps the limb to move.
There is thus a causal chain which transmits information from a sense organ to an organ capable of making decisions, and onwards to a motor organ. In this respect, the model is analogous to a modern understanding of information processing such as in sensory-motor coupling.
Inheritance
Aristotle's inheritance model sought to explain how the parents' characteristics are transmitted to the child, subject to influence from the environment.
The system worked as follows. The father's semen and the mother's menses have movements that encode their parental characteristics. The model is partly asymmetric, as only the father's movements define the form or eidos of the species, while the movements of both the father's and the mother's uniform parts define features other than the form, such as the father's eye colour or the mother's nose shape.
Aristotle's theory has some symmetry, as semen movements carry maleness while the menses carry femaleness. If the semen is hot enough to overpower the cold menses, the child will be a boy; but if it is too cold to do this, the child will be a girl. Inheritance is thus particulate (definitely one trait or another), as in Mendelian genetics, unlike the Hippocratic model which was continuous and blending.