Originally posted by rogue06
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If instead of an individual tree you speak of trees in general, then you are talking about a (Aristotelian) species or genus, which has an essence: the set of properties that are true of every tree. And of course the essence of a species cannot change. If you changed the essential properties then you would be talking about a different species. Or if all trees became extinct, that wouldn't destroy the species. It would just mean that there no longer exists any individual of that species.
For example, suppose modern tigers are the descendants of saber-toothed tigers (I don't know whether that is true, but just suppose for the sake of argument). That doesn't mean the species (in the Aristotelian sense) "saber-tooth tiger" changed into the species "modern tiger". The species "saber-tooth tiger" still 'exists' (has its essential definition), because we can still predicate true things about it, such as, "Saber-tooth tigers are extinct." In that sense the essence of a species (like "saber-tooth tiger") is immutable.
Did Aristotle think that no individuals of a new species could come to be? I don't think so. 'Sculptures of Socrates' is a species, and before the first one was sculpted, there had never been any.
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