Aristotle was the first biologist. Sadly, even though around one-third of his extant writings are devoted to living things, few of his interpreters take the biological basis of Aristotle's philosophy very seriously. This despite the fact that Charles Darwin said in a late letter that his "two gods" Linnaeus and Cuvier were "mere school-boys to old Aristotle"! It's a shame that Darwin didn't live long enough to study and comment on Aristotle's biological writings.
One of Aristotle's deep insights into living organisms is that they act for their own sake, in two senses. First, an organism's activities benefit it. Second, its activities serve a purpose within the organism's way of life. This two-fold "for-the-sake-of" relationship applies exclusively to living organisms, because only by staying alive do they remain in existence, and the process of living simply is activity. (Contrast such a living organism with a natural but inanimate object; for instance, we can say that a lake is full of life, but the lake itself is not alive and it does not act for its own sake - only the plants and fishes and birds that live in and around the lake do that.)
Here I make a digression into the topic of causation...
Although this "for-the-sake-of" relationship is in fact quite straightforward and scientific, throughout intellectual history there has been great confusion about it. With regard to the beneficiary of action, some interpreters have claimed that only the organism itself can benefit, a kind of psychological egoism that is entirely absent in Aristotle (e.g., in his biology reproduction is a core instance of action that benefits an organism and in his ethics the shared activities that feature so prominently in love, friendship, and community are the highest applications of personal virtue).
With regard to activities serving a purpose within an organism's way of life, medieval notions of so-called final causation (a term Aristotle never uses) have misled interpreters into believing that Aristotle claimed future states could "reach back in time" to cause earlier events. Similarly, medieval notions of teleology (another term Aristotle never uses) have led interpreters to attribute consciously held purposes to all living organisms, even plants and pre-conceptual animals; failing that, such interpreters have believed that such purposes must exist somewhere, so they've attributed them to an all-powerful creator.
Yet there is no more mysticism in saying that an activity "serves a purpose" within an organism's way of life than there is in talking about "forces" in physics or "bonds" in chemistry. Consider, for example, the play behaviors of nearly all mammals and some other animals. As professor Peter Gray and others have explored in depth, such behaviors serve the purpose of preparing young animals for the physical, perceptual, and social challenges of adulthood. But an adolescent fox, for instance, doesn't form the conscious purpose of self-improvement before engaging in play behavior, nor does a mature fox cause its earlier play behaviors through some sort of backward causation.
Returning to the main thread, the conditional nature of existence for living organisms has significant implications for biology, psychology, and human ethics. The impact varies depending on which sense of "life" we're talking about, because in ancient Greek there were (at least) three terms of interest:
I'll explore these implications more fully in future journal entries, and in my forthcoming book Complete Yourself: Aristotle on Human Fulfillment.
(Cross-posted at philosopher.coach.)
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