Biology was a natural pursuit for Aristotle, given his family's medical background. Along with his achievements in logic, his work in biology constitutes his greatest and longest-lasting success. He identified approximately 495 different species of animals, some of which were alluded to briefly and others that were studied in considerable depth. A significant part of his achievement was simply the sheer amount of data that he collected, but he is also praised for the skill and care with which he organized the data, along with the insights he offered.
For example, he recognized that the cetaceans possessed mammalian characteristics–a fact that all other writers overlooked until the sixteenth century. He showed great accuracy in his depiction of the chicken embryo. Such achievements might mean little for the lay reader, but later generations of biologists have expressed great admiration for the level of depth and accuracy that he attained. There is no doubt that Aristotle was far ahead of his time.
His central work in biological studies was titled The History of Animals. Aristotle draws the most important distinctions between animals with and without blood and between viviparous (reproducing offspring within the female's body, as generally the case with mammals) and oviparous (reproducing through the hatching of eggs) animals. He paid considerable attention to the questions of reproduction and heredity, determining what factors contribute in what ways. Aristotle's teleology played a particularly important role in his biological studies. He believed that no organ was given to an animal without a purpose. Thus he was careful to distinguish between final and variable characteristics. Final characteristics were those essential to an animal species, while variable characteristics consisted of qualities that develop rather than being naturally endowed.
For Aristotle, biology and psychology were intertwined, much more so than we would view them today, and he treated the two subjects as one science. The purpose of psychology was to discover the attributes and essence of the soul (translated from the Greek work psyche). Aristotle struggled to come up with a single definition of the soul and concluded that none existed. On the other hand, the variations in the kinds of souls were not so different that some common ground could not be ascertained. Aristotle therefore arranged a series of various forms that become increasingly complex, so that each form of the soul possesses the qualities of all those that precede it in order. The most basic soul is nutritive, which exists in all living things, including both plants and animals. Beyond the nutritive soul is the sensitive soul, possessed by all animals. This category can itself be broken down into the same kind of hierarchy, in which touch is the most basic sensation. A sensitive soul is capable not only of perception but also of desire, since it can feel pleasure and pain. Moreover, an animal can possess two additional faculties that are not necessarily found in all: the first he calls imagination, which also includes the faculty of memory and is an extension of the cognitive aspect of the animal; the second is the faculty of movement, an extension of the appetitive side. Human beings of course possess the most complex soul, which exercises the faculty of reason. Aristotle justifies this hierarchy by showing that the faculties are ordered by their necessity, nutrition being the most fundamental and reason contributing not so much to sustenance as it does to well-being.
One key issue that Aristotle raises is the relationship between soul and body. He views them as inseparable and makes the analogy that the soul is to the body as form is to matter. In other words, the soul is the primary actuality of the body, providing the body with its essential character and therefore is inseparable from it.
Aristotle's account of such faculties as common sensibility and imagination generally reveal the limitations of his knowledge of physiology. In particular he is concerned with the faculty of perception, which is linked most closely to sight but has a more general application that plays a role in all judgments related to the senses. Aristotle also wrote a series of supplemental treatises on the difference between memory and recollection, the acts of sleeping and waking, the act of dreaming, and more. But these are read generally only for historical interest. While Aristotle may have viewed biology and psychology as one science, the respective lasting significance of each demonstrates the vast difference between them. Aristotle's psychology is based largely on speculation that has since been discarded based on improved understanding and technology, while his contributions to biology were grounded in skilled observations interpreted with keen insight that took centuries to surpass.