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_.

By this abrupt contraposition of body and mind as reciprocally independent