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Esse substantiæ non dependet ab esse alterius sicut ei inhærens, licet omnia dependeant a Deo sicut a causa prima.”—St.

It is a very important axiom of the schoolmen in this case—that, a posse ad esse non valet consequentia, you can draw no inference from the possibility of a thing to its reality, but that, in the reverse order, ab esse ad posse, the inference is inevitable: if it is, or if it ever has been—then of necessity it can be.