Aristotle points out in Book I, chapter 1 that a plurality of natures is a necessary condition for substantial change to be able to occur. Why, because new substances/natures come into existence when things of different kinds are combined: "... when things are combined htere is coming to be, and when they are dissolved there is perishing." (314b8)
It's obvious that this would (rightfully) seem to be the case at Aristotle's time, for organisms are always made of non-uniform parts. But we could say the same today on the basis of what we know about atoms, they differ in kind because they consist of different combinations of parts.
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