Galen, On the Natural Faculties 2.3 is a refutation of pores. If we had pores, Galen argues, then they would have to grow in proportion as the rest of us grew. Again he invokes the children’s game of blowing up pig bladders, and again the question circulates around how to distinguish between real and (hypothetical) artificial life.
His interlocutor in this chapter is Erasistratus, one of the two medical bogeymen he confronts throughout OTNF. Erasistratus tries to construe biological processes in a mechanical way as much as possible, while Galen favors an account that imbues the body with various extramechanical powers (dunameis) of attraction, separation, transformation, etc. An example of the distinction: Erasistratus thinks that the kidneys operate like a sieve, pulling thicker matter out of the blood, while Galen thinks that they draw serous humor and various other excrements toward themselves like magnets attracting iron. Given the way that circulation was understood to work in the first century CE, Galen can make a strong case, but that’s neither here nor there.
Erasistratus’ mechanical explanations usually involve his postulating the existence of invisible (we would say microscopic) pores in the body that allow some substances to enter some spaces while excluding others. If this were so, says Galen, then the same pores would have to exist in children, or even in embryos, as exist in adults. To keep up with the functional demands of a growing organism, the pores would also have to grow, so that we as adults would be walking around full of gaping holes. Galen insists on the topological conformity of the body: just as a certain region on a pig’s bladder grows when children inflate it, so must all regions on the body grow in proportion as the whole body does.
One might ask: instead of growing the holes that are already there, why not just add more holes? Well, says Galen, it’s important that we distinguish living bodies from the products of human craft – say, a basket. Baskets get larger through the addition of more fibers, but bodies get larger by growing, uniformly (the pig bladders, again). Galen asks us to envision a basket with a digestive tract. We can easily imagine such a bucket getting longer at one end or the other, but how would the whole thing get larger in a proportional way as human bodies do? The basketweaver would have to undo the whole thing and remake it at a larger scale. It follows, says Galen, that the human body doesn’t grow by the addition of new parts, basket-style, but according to a natural power of growth that exceeds, dimensionally, the artistic power of a Phidias or a Praxiteles.
Galen has given us another recipe for artificial life, this one strangely close to how we now understand growth to work: when the cells in our body divide, they add extra parts just as a basketweaver would do. In this instance, by comparison with Galen’s concept of the person, we’re robots.