On De Anima or On the Soul (350 BCE), books 1 and 2. We lead off with listener mail; Aristotle starts 18 minutes in.
What can this ancient text tell us about biological life? Much of the interest and difficulty here is the very different conceptual schemes at play: What counts as a scientific explanation? Aristotle gives a highly general account about what, for all living things, makes them alive. He describes life as "the first actuality of a natural body which has organs," meaning that bodies (our bodies, plant bodies, animal bodies) truly express their nature only when they're growing and reproducing and all that stuff that bodies do. The body itself is potential, and life is its actuality. So what the heck kind of explanation is that, and how does it tie into all that metaphysical stuff you may have heard about Aristotle concerning forms and different types of causation and substances?
Seth returns to complete the formal foursome as we slowly penetrate this difficult text. With this one more than others, you'd be well advised to follow along in the text. This online translation by D.W. Hamlyn should do the trick. The line-by-line commentary that Seth refers to is by Eugene T. Gendlin.
Aristotle picture by Genevieve Arnold.