The question whether the idea of space is conditionally or unconditionally necessary can only be determined by the solution of the deeper question whether space is a real entity or a relation. If space is a real entity, it must have properties or attributes, but what philosopher of any reputation has ever attempted to set down the properties or attributes of space? They who assert that space is an uncreated, independent, and indestructible entity, ought to be able to define it and tell _what_ it is. Dr. Porter tells us that space can not be defined, "We can not form a concept of this _entity_ by means of generalized attributes or relations."[71] Can that be for us an entity of which we can form no concept, and which we can not determine in thought by any attribute or relation? The writer of the article on "The Philosophy of Time and Space," in the _North American Review_,[72] is an earnest defender of the objective reality of space as an independent and indestructible entity, and he has defined and analyzed the concept. "Space is _absolute vacuity_" (p. 91). "The idea of space is a triple synthesis ... of three _negative_ notions--receptivity, unity, and infinity; the first is the negation of matter, the second is the negation of divisibility, the third is the negation of limitation" (p. 95). Do these words convey any knowledge? Absolute vacuity is void, empty, inane. Absolute vacuity is pure nothing, and of course there is nothing to be divided and nothing to be limited. Absolute vacuity is a negation, and unity and infinity are negations of a negation--that is, they are predicates of nothing. "Negative notions" must be predicates of _something_, otherwise they are a mere negation or absence of thought, and convey absolutely no knowledge. We may, if we please, assert with Hegel, that "Nothing is the same as Being," and then amuse ourselves with making affirmations concerning vacuity, nihility, and unreality to the disgrace of philosophy; but the common-sense of mankind will repudiate our absurdities. We can not think about nothing; all thought must be positive. Thought must have an object, and that object must be either an entity, or the attribute of an entity, or a relation between entities.