Inductive Generalizations of Horses

Horses are homogeneous. I can learn about it inon terms of horses that have already been studied, and i will generalize well to horses that have not themselves yet been studied. And this is, i don't mean this to be a pun, i need the category membership to be stable. What's like, what's in the category, the kind of things that are in the category should be stable. Shouldn't be constantly shifting or changing. It will generalize. So horses are, in really important ways, homogeneous. That's why we can have veterinary medicine and things like that.