Near the end of On the Natural Faculties, at a point when Galen feels he has already proven his case, he stops to sum up. The various parts of the body receive and process food not mechanically, like cogs in a machine, but because they have “natural powers” (phusikoi dunameis) through which they attract, hold, and digest the nutriments proper to them. The same natural powers drive each organ to expel what it cannot digest (its perittoma) and to supply the results of its digestion to the rest of the body.
Nature thus governs each organ in much the same way as it governs the whole organism. This claim, however, introduces complications of its own, because it blurs a distinction between organ and organism that Galen, for reasons of his own, wants to maintain. If both organs and organisms are what they are – i.e., alive – because of a set of natural powers, what justifies our subordinating the one to the other in the course of our analysis? What prevents us from treating organs as organisms, and vice-versa?
Galen tries to remedy this breach by asserting a distinction that will strike modern readers as a little strange. It’s true, he admits, that the organs have appetites (orexeis) for their nutriments just as the body itself has an appetite for food. What organs lack, however, is a sense of their own teleological reason. The stomach, to use Galen’s example, desires food for its own sake and not in order to provide fitting nutriment (epitedeia) for the other parts of the body. If it did act with such an intention, he goes on, it would no longer be a phusikon organon (a natural organ? an organ by nature?) but some kind of animal (zoion ti) which use mind (noun) and reasoning (logismon) to decide (hairesthai) on the best course of action. In effect, what sets organs apart from organisms is that the former lack what a philosopher would call (and Galen comes close to calling) prohairesis, the ability to choose its activity on the basis of a rational plan. Organs act by instinct alone.
In light of the resonances of his argument, it is odd that Galen claims prohairesis would convert an organ into “some kind of animal.” As Galen must have known, most philosophers held that non-human animals precisely lacked prohairesis: this was what distinguished brute beasts (ta aloga) from humans. By this logic, the organs that Galen describes – equipped with their own natural powers and appetites, with what amounts to an instinctual program – are already animals. The best we could say, to save Galen’s account, is that they are animals of a particularly tame sort whose instincts never drive them to rebel against the body of which they are said, without apparently much justice, to be mere parts.