As we have seen, Greek thought before Aristotle tended to fall into two broad camps on the nature of life. One (the Milesians and Atomists) held that life could be explained more or less naturally based on the parts of organisms, and the other (the Eleatics and Pythagoreans) that empirical evidence was insufficient, and that the true nature of living things arose from more abstract principles.
Into this scene came Plato, or rather, Socrates, who we know almost entirely through Plato, and Plato’s ideas himself. In the Platonist account of Socrates, the focus of philosophy was to find wisdom and justice and the good life. As a result, there is little actual interest in the living world by Socrates, and by Plato himself, other than as a backdrop to these human-centric arguments on knowledge, ethics and gods, etc. Plato was, however, interested in some metaphysical questions that do have a bearing on biology, effectively changing how it was conceived for two millennia.
According to classicist David Sedley, in a 2009 book Creationism and its Critics in Antiquity, Plato, and probably Socrates (who left no writings of his own), made explicit an assumption that had been generally uncontentious in the pre-Socratics, that the universe was made along rational lines by a creator (in Plato, a demiurge, who was not the highest deity, but one who acted like an artisan constructing the material world along the design of the most distant and high mind of the God). In short, the world was designed.
Of course, that in itself was not new – religious aetiologies had always had the idea in mind that gods made things deliberately; as Sedley notes, the view occurs in Hesiod’s Theogony, a contemporary of Homer (8th century BCE). What was new was Plato’s rationale. Instead of the hotchpotch rationales given for the creation of the world or humanity in most religious folk stories, Plato, through his character Timaeus (in the dialogue by that name) offers the story that the god who created the world made it so it would represent the soul of reason, as that was what the god considered was good. Thus, the universe was spherical, because movement would be eternal if it moved in circles. The problem of change was thus solved – change is (at best) eternal and thus not really change.
For the Greek philosophers, as we shall see with Aristotle, movement requires a motive force, which they called psuche, and which is usually translated (misleadingly) as soul. The universe according to Plato has a “world soul”, created by the Demiurge. It needs nothing more than this to be in motion. Human beings were created to mirror this world soul, and so they have a head in which the soul exists (contrary to Aristotle, who thought the brain was merely a cooling device). But as animals, including humans, are part of the universe rather than the entirety of it, they need to have limbs to stop them rolling around (seriously). So they have four limbs. And as they are not self-contained, as the universe is, and are constituted by the four elements (Plato adopts the Emepdoclean four elements theory as a given), the human body also need to have a nutritional set of parts, and so we have a torso as well. Other than that, the human body is an echo of the universe. [See Karfík 2012 for a good summary of this.] This opinion came to be known later as the microcosm/macrocosm theory – as above, so below.
Plato’s cosmology and anthropology relies upon his epistemology. According to him, knowledge is not gained through observation, but by remembering (anamnesis) what was in the soul before birth, and so knowledge of the physical world is a poor substitute for real knowledge gained from the intellect itself. In fact, the physical world is not really real, but is a mere echo of the world of Ideas or Forms (Greek: eidei). The pre-eminence of Forms over actual bodies is a long-running theme in biology thereafter, and continues the Eleatic tradition.
Karfík F. (2012), “The Constitution of the Human Body in Plato’s Timaeus”, Croatian Journal of Philosophy Vol. XII, No. 34.
Sedley, D. N. (2007). Creationism and its critics in antiquity. Berkeley, CA; London, University of California Press.