Aristotle has this trenchant comment to make in the Physics:
That nature exists, it would be absurd to try to prove; for it is obvious that there are many things of this kind, and to prove what is obvious by what is not is the mark of a man who is unable to distinguish what is self-evident from what is not. (II.1)
Aristotle firmly holds that kinds exist: or, to use a less obvious term, he believes in forms or universals (though not in the way that Plato does). More than that, he considers their existence to be so obvious as to be self-evident. Why so, since others disagree with him? Well, we risk plunging into the absurd, as he says, by even asking the question. But let us forge ahead for a bit and consider what Aristotle might have in mind. When he says there are many things of this kind what he is talking about is the fact that there are many trees of the same kind (oak, maple, sycamore), many lions, many tigers, many bears, and so forth. If these things weren’t all of the same respective kinds, it would be impossible to talk about kinds of things at all. It would also be blatantly contrary to the evidence of our eyes. That is why The Philosopher says it would be a waste of time to try proving that kinds exist. It’s so immediately obvious that it is self-evident.