Philosophers, when speaking of things as existing in their natural state and condition, are wont to say indiscriminately that substance is a being which “exists in itself,” or a being which “subsists by itself.” This they can do without any danger of error so long as they keep within the bounds of pure nature; since, in the natural order, anything that exists in itself subsists by itself, and _vice versa_. But natural things can, by supernatural interference, be raised to a mode of existence transcending their natural condition, as we know by divine revelation; and in such a case, the mode of substance and the mode of the suppositum must be, and accordingly are, most carefully distinguished from one another. Thus we know by faith that in Christ our Lord there is the true substance of a human body and of a human soul; and nevertheless we know that his human nature does not subsist by itself, but by the Divine Person of the Word. The obvious inference is that a nature which exists in itself does not necessarily subsist by itself; in other terms, the formality of substance and the formality of the suppositum are entirely distinct from one another, and the one can remain without the other. “What makes substance to be essentially a substance,” as Suarez remarks, “is not its subsisting _actually_ by itself, but its having an essence to which subsistence is naturally due—viz., an essence which is of itself a sufficient principle of subsistence.”(1) From this we learn that the words _per se esse_, or “to subsist by itself,” are inserted in the definition of substance, not to show what substance as such is, but only to point out what is naturally due to substance—viz., what accompanies it in its natural mode of existing. Substance as such would therefore be sufficiently characterized by the words, “that which is in itself.” Let us now inquire what is the legitimate meaning of these last words. A thing is said to exist in itself which not only has in itself what is needed for its own sustentation, but is moreover actually unsustained by anything lying under it, while it is itself the first subject of all its appurtenances. Such is the legitimate and traditional meaning of the words, “to exist in itself.” Hence substance may be legitimately defined as “a being which by its intrinsic constitution has no need of being supported by a subject, and which is not actually supported.”