From Joe Sachs’s introduction to On the Soul:

The inner life of the animal presents itself to us in its outer activity, and teaches us that we too dwell innately in our bodies. When the bird flies away, and doesn’t bump into the branches and other obstacles it glides around, we recognize two things: that the bird itself originated its motion, and that it guides its course by means of perceiving the things around it. Living motion reflects an inner source, just as living perception requires an inner awareness. … The two unities in the experience of the living body are one and the same, one soul. …

The structure of this world, not some other that we may imagine or invent, but this one that we experience, must accommodate the structure of the natural and that means, pre-eminently, the living. …

What is that structure?  Aristotle traces it down many roads. In the Platonic dialogues, Socrates is always asking the question ti esti?, what is it?, about any single look that makes many perceptible things all be the same one thing. It is possible to treat this as a logical question, having to do with classifying the world, attaching universal names to particular things. But we can sort the world into any classes we please. Aristotle is interested in the way the world sorts itself out, and this is visible in anything that keeps on being the same while constantly undergoing change. And what if that enduring sameness is of a kind that could not even be present except in the course of change?  In that case, the thing does not hold out passively against change, but absorbs change into itself, molds it into a new kind of identity, a second level of sameness, a higher order of being. For such a being, to be at all depends on its keeping on being what it is. Aristotle sums up this way of being in his phrase to ti ên einai.…

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