are not, for all their smallness, indivisible atoms—the latter being smaller still—one may either infer what is unknown about perceptible things from what has been observed in them, drawing an analogy between what eludes perception and what is perceptible. This is among the clearest and soundest forms of analogy: comparing parts alike in their essential nature with one another regarding what holds for that nature. Differences in size do not entail differences in their essential nature and attributes. Alternatively, one does not say that what holds for that indivisible part is unknown.

As for saying that things differing in essential nature must have parts that, while sharing the essential nature of their respective wholes, are all alike in essential nature—this is something whose falsity is immediately evident to reason. The proponent of this argument has posited parts differing in essential nature and said that decomposing their composite must ultimately yield small, indivisible parts. We reply: Granted that this is so, it still cannot be said that the parts of bodies differing in essential nature must be alike. Rather, to say that they must differ, because a part of a body shares the essential nature of its whole, would be more correct and more deserving of acceptance than to say that the parts of bodies differing in essential nature must themselves be alike in essential nature.

This is why those who maintained that bodies are alike also maintained that atoms are alike, while those who maintained that bodies differ also maintained that atoms differ. Either it is said that atoms are alike in essential nature, but