Our ideas represent in part things, in part qualities.  Substance is defined by the concept of independence as res quae ita existit, ut nulla alia re indigeat ad existendum; a pregnant definition with which the concept of substance gains the leadership in metaphysics, which it held till the time of Hume and Kant, sharing it then with the conception of cause or, rather, relinquishing it to the latter.  The Spinozistic conclusion that, according to the strict meaning of this definition, there is but one substance, God, who, as causa sui, has absolutely no need of any other thing in order to his existence, was announced by Descartes himself.  If created substances are under discussion, the term does not apply to them in the same sense (not univoce) as when we speak of the infinite substance; created beings require a different explanation, they are things which need for their existence only the co-operation of God, and have no need of one another.  Substance is cognized through its qualities, among which one is pre-eminent from the fact that it expresses the essence or nature of the thing, and that it is conceived through itself, without the aid of the others, while they presuppose it and cannot be thought without it.  The former fundamental properties are termed attributes, and these secondary ones, modes or accidents.  Position, figure, motion, are contingent properties of body; they presuppose that it is extended or spatial; they are modi extensionis, as feeling, volition, desire, representation, and judgment are possible only in a conscious being, and hence are merely modifications of thought.  Extension is the essential or constitutive attribute of body, and thought of mind.  Body is never without extension, and mind never without thought—­mens semper cogitat.  Guided by the self-evident principle that the non-existent has no properties, we argue from a perceived quality to a substance as its possessor or support.  Substances are distinct from one another when we can clearly and distinctly cognize one without the other.  Now, we can adequately conceive mind without a corporeal attribute and body without a spiritual one; the former has nothing of extension in it, the latter nothing of thought:  hence thinking substance and extended substance are entirely distinct and have nothing in common.  Matter and mind are distinct realiter, matter and extension idealiter merely.  Thus we attain three clear and distinct ideas, three eternal verities:  substantia infinita sive deus, substantia finita cogitans sive mens, substantia extensa sive corpus.