S. Thomas, as we might expect, teaches the very same doctrine. “Substance,” says he, “is a thing whose quiddity requires to exist unsupported by anything else”—_cui convenit esse non in alio_;(7) and he adds that this formality (_esse non in alio_) is a mere negation; which is evident. And in another place, “_Substance_,” says he, “does not differ from _being_ by any difference which would imply a new nature superadded to the being itself; but the name of substance is given to a thing in order to express its special mode of existing.”(8) Two things, then, or two constituents, are needed, according to S. Thomas, that we may have a substance: a physical being and a special mode of existing. The physical being is a positive reality, a nature perfectly constituted, both materially and formally, whilst the special mode is a mere negation; but, though a mere negation, is that which causes the thing to be a substance, as _the name of substance is given to the thing in order to express its special mode of existing_. Therefore the thing itself apart from such a special mode cannot be a substance, any more than a six‐pence apart from its rotundity can be a circle. Toletus includes in his definition of substance both _the thing_ and _the special mode_ of existing. He says: “The first substance is a sensible nature which is not predicated of any subject nor exists in any subject.”(9) Suarez says even more explicitly, “It is not necessary for the _essence_ of substance that it should have its own subsistence, but that it should have the mode of substance.”(10) We cannot, then, overlook, and much less discard, this special mode without destroying the _essential_ notion of substance as such. Now, he who defines substance to be simply a thing which has _no need_ of support overlooks and discards this special mode; hence he destroys the essential notion of substance as such. Balmes, in his _Fundamental Philosophy_, says: “In the notion of substance, two other notions are implied—to wit, that of permanence and that of non‐inherence. Non‐inherence is the true formal constituent of substance, and is a negation; it is grounded, however, on something positive—that is, on the aptitude of the thing to exist in itself without the need of being supported by another.”(11) This passage establishes very clearly the common doctrine that the aptitude of a thing to exist without being supported is not the formal constituent of substance, but only the ground on which the proper formal constituent of substance (non‐inherence) is conceived to be possible. Ferraris, a modern Italian Thomist, in his course of philosophy, says explicitly that substance is destroyed if its “perseity”—_per se esse_—be taken away.(12) The word “perseity” stands here for the “special mode” of S. Thomas, the “mode of substance” of Suarez, the “non‐inherence” of Balmes, etc.