Plato’s pupil, and later Alexander the Great’s tutor, Aristotle was born in Stagira in Macedonia in 384 BCE (died 322 BCE), and is sometimes known as the “Stagirite” in older writings. His activity was mostly in Athens, where he studied with Plato, and where he set up a school after Plato’s death, where he and his students would walk and talk in the shaded walks around the gymnasium known as the Lyceum. His school is thus called the “Peripatetics” (“those who walk from place to place”).
Aristotle’s works are divided into
- the logical (the Organon, including the Posterior Analytics and the Categories – see below),
- the metaphysical (the term “metaphysics” itself possibly comes from the position of his works under that title in the order of later bindings – it came after (meta) the Physics),
- the physical (including the works involving biology),
- the psychological (including On the Soul, or De Anima),
- the ethical and
- several contentious works under his name dealing with poetry and rhetoric.
The works that apply to his very influential ideas on life, and natural history are often called the Liber Animalium (Book of Animals), but which includes three books (known in their Latin titles: Historia Animalium, De Generatione Animalium, and some minor works like De Locomotio Animalium and so on).
Aristotle’s biological ideas are founded firmly on his theoretical philosophy and physics, and so before we get into his works on life (including human nature) we have to understand some of his teachings. It is important to know that he considered the study of the natural world to be natural philosophy and the study of the terrestrial (including the living) world came to be known as natural history after his book Historia Animalium, as historia in Greek originally meant “investigations”.
Unlike Plato, Aristotle believed that the foundation of knowledge was experience. He considered logical issues were important, however, so that we could explain why things (like humans lacking tails, to use James Lennox’s example, 2001, p 13) had characters they did because they were part of a larger class of things that had those characters (in this case, hominoid apes, which lack tails). Knowing the class, which he referred to as genera (sing. genus) allowed us to understand the properties of a member of that genus.
A genus is like a set (not coincidentally, since the Aristotelian categories informed modern set theory). Genera can include individual things, like John Wilkins, or subgenera like Australians. A subgenus is a species of that genus (and no species is properly a member of more than one genus in what Aristotle considers a well-formed classification). However, unlike the modern Linnaean taxonomic scheme, a species can also be a genus of something else, and have its own subgenera or individual members.
As we shall see when we come to his biology, Aristotle did not always use the Greek terms genos (Latin: genus) or eidos (Latin: species) when discussing living kinds. However, the terms of his logic (in the Posterior Analytics, for example) were in the first instance applied to terms (“predicates” via Latin, translating the Greek katēgorēma, from which we get “category”). In short, this was explanation in terms of words. And this was entirely proper for the explication of logic. However, it is not how he undertook his natural history, as we shall see.
Aristotle also had a theory of categories (in the Categories, appropriately enough), which was an a priori classification of being. There were things that were accidental, and things which were essential. There were also things that were “universals” (katholou, a portmanteau word meaning initially “according to the whole”; we might use the world “class” to express it today) and things which were particulars (individual things). This gives us four combinations (a number Aristotle was very fond of):
- accidental universals,
- accidental particulars,
- essential universals and
- essential particulars.
“Essential” here means “what it is to be” something (literally, in the Greek). So we speak of things as being accidentally or essentially particulars or universals. This is relevant to his treatment of classification and definitions of living things.
Aristotle’s theory of physics included the Empedoclean four-elements theory. He held that there were two contraries: hot-cold and wet-dry. Each element was a mixture of these properties. There was also a fifth element, later called the ether (aither) which the heavens, which did not corrupt or change, was made of. The heavens began at the lunar sphere, and generation and corruption (coming to be and passing away) only occurred in the sublunary realm, which included the earth. Aristotle offered up a “two sphere” universe:
The earth was at the centre of the universe and the planets sat on crystal spheres. The outer shell was the sphere on which the fixed stars were, well, fixed. Change in the heavens was circular and eternal, as Plato had thought, while change in the sublunary realm was temporary. Later Christian theologians added God’s habitat, heaven, outside the fixed sphere.
Terrestrial things were composed of mixtures of the four elements, and a surplus or deficit of them from the normal (“proper”) balance caused all kinds of difficulties such as illness or deformed animals, including humans.
The best introduction to Aristotle’s philosophy of biology is
Lennox, J. G. (2001). Aristotle’s philosophy of biology: studies in the origins of life science. Cambridge, UK; New York, Cambridge University Press.