The theory describes five major biological processes, namely metabolism, temperature regulation, information processing, embryogenesis, and inheritance. Each was defined in some detail, in some cases sufficient to enable modern biologists to create 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 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.
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 made 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.
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, εἶδος (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.