Essence, Genus, and Species

The essence of a thing is what makes it what it is. If you change a thing’s essence, you have a different kind of thing. A thing’s essence is distinct from a thing’s accidents, those features of a thing that can be changed without changing the kind of thing it is. If I’m not mistaken, a thing’s essence is also referred to as its substantial form, though there might be a subtle distinction that eludes me.

Thomas often speaks of essence in terms of genus and specific differences, or genus and species. These terms are similar to those used in zoological taxonomy, though I suspect that here as well there are subtle distinctions that elude me. In particular, I’m not sure if Thomas thinks that every being has a species and a genus and that’s it, i.e., that his system of taxonomy is only two levels deep (which is hardly enough), or whether it’s simply a convenient way of expressing similarities and differences between two begins which have both essential similarities and essential differences.

My suspicion is that both are true: that Thomas presumes, God’s creation being an orderly place that reason is capable of understanding, that in principle every being does have a proper genus and species, but that in practice he doesn’t try to enumerate them. Though it may be that Aristotle provides such an enumeration which Thomas takes for granted. Aristotle seems to have been very fond of the making of lists and the categorization of things (and of the definition of the categories by which to categorize them).

Note: If anyone can recommend a good introduction to the thought of Aristotle that is accessible to the layman but more advanced than Mortimer Adler’s Aristotle for Everybody, I’d greatly appreciate it, as I’ve not been able to identify one. I do highly recommend Aristotle for Everybody, though.

Aristotle for Everybody

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