Aristotle's "De Anima" was all about creating a third option for analyzing natural phenomena that didn't rely on either the sophists / pyrrhonists slipperiness or any kind of metaphysical essentialism. This breezy pluralism is what has proven attractive to many people even since we've learned that the atomist metaphysicians were, in fact, correct all along.
One of the standard challenges to the definition of any living thing would be eating: a deer, say, survives by eating grass. ...
Rationally, then, where does the deer end and the grass begin? Many contemporary philosophers are happy to let this land as a nice spot of ecological woo-woo and feel all wooly-headed. Likewise, both of the non-Aristotelian crews I referenced would be happy to assert that there was really no deer and no grass in the first place, and we draw pragmatic distinctions between grass and deer as it suits our purposes. Not Aristotle, however. Let us cleave the deer from the grass!
Aristotle's solution for this kind of thing is to introduce the idea of potential: the deer might eat grass, to be sure, but the deer might also survive on grain, apples, and the rest. The essential deer-ness of the deer has something to do with a deeper potential to transform deer snacks into fully functional deer. This has a more general application, too. Aristotle got to respond to the question of identity writ large by insisting that even when some identifiable whozit acts unusually...
Beneath the funny hat and the fake mustache we find that there is, in reality, a deeper potential beneath the immediate behavior. This is the essence of the thing: a kind of potential or capacity that holds together lower-order potentials and capacities.
If Socrates is a husband AND a teacher, that simply reveals that whatever Socrates is in essence must also include the capacity to teach and the capacity to couple. And so on with other examples.
Aristotleans linked this to other observations about nature: humans seem to have lots of potential, whereas ducks seem to have less, and mossy rocks seem to have barely any potential at all. If you shave a man he's still a man, but if you shave a mossy rock it ceases to be a mossy rock.
So this leads us to the distinction between mineral, vegetable, animal, and rational potentials. A mineral really only has one potential, which is the potential to exist. While a vegetable also shares that...
A vegetable also has the potential to develop in complexity and organization. Call this maturation. And the incredible thing about vegetable maturation is that they seem to mature according to kind. Aristotle never saw a carrot get confused and accidentally develop into an apple tree, so it seemed to the Aristotelians that these paths to maturity were stable, predictable, locked-in runways leading from present to future. Call this the "telos," or "goal" of development.
Cracking stuff, these animals. They have behavior! And behavior seems like a higher kind of capacity than the mere maturation that vegetable life has, because the animals also mature but they might mature in any number of positions, etc, with many more complex possibilities available to the animal.
The animal that stands above all the rest, however, is the human animal. But not just any human animal, because Aristotle might suspect that slaves, women, barbarians, and the disabled wouldn't count
A properly HUMAN human would of course be an educated adult man with leisure time, like Aristotle. And what is it that Aristotle does with his leisure time that makes him such an excellent example? Why, he REASONS. He moves from specific observations and general categories and back again. And of course since Aristotle's teacher wouldn't stop banging on about the neo-Pythagorean forms that supposedly make logical thinking possible, Aristotle integrated these forms into his understanding, too.
Now with this handy distinction at hand, we might ask "what makes the world so?" and Aristotle would answer,
- The mineral stuff is so because of its material substance. It merely exists, and so if there's going to be any change or development concerning it, that should follow simply from the tendency of EG clay to take an impression.
- The vegetable stuff is so because it is striving after its telos. The plants want to achieve their mature organization, and they'll reshape themselves for it.
- The animal stuff is so because it is influenced. Dogs, horses, pigeons are all trainable, and all that training indicates that animals probably obey patterns of stimulus and reward. Of course they'll also strive after their own maturation, but if you really want to understand why an animal keeps doing such-and-such behavior you should really look at what has facilitated or incentivized the animal to do it.
- As for these human animals, the properly rational ones (like Aristotle) are such-and-so as they are because they are busy thinking rationally about triangles and stuff. Slaves, women, foreigner, disabled people need not apply.
You have a four-level natural world with four kinds of causation at each level:
- a mineral world governed by material causation
- a vegetable world governed by teleological causation
- an animal world governed by efficient causation
- a world of ideas governed by formal causation
Thus Aristotle dispensed with all of the naysaying sophists, pyrrhonists, and metaphysicians for a millennium and a half... until the Democritan metaphysicians were proven right and efficient causation became the exclusive paradigm sufficient to describe the sum total of the natural world.
By the by, from this it's pretty plausible to believe in a "world soul," or to describe Nature itself as this vast and comprehensive process of development which comes from itself and moves into itself infinitely. Its potentiality is apparently infinite, as it gives rise to all of the potentialities of every mineral, vegetable, animal, and rational being.
This world-soul was also what the stoics, especially Chrysippus, identified as the supreme rational principle of all reality.
So vegetable life has the potential for teleological maturation, and animals do too. But animals go one step beyond vegetable life because animals have action. An animal can be east of the river in the AM and west of the river in the PM. The pre-Socratics thought that this was quite a stumper for the definition of identity, but Aristotle's system seems especially geared to resolve this: the animal is neither a thing in the east nor a thing in the west, but is a thing with the POTENTIAL to move.