become extended, or matter think. But to Descartes the dualism is
absolute, because it is a presupposition with which he starts. Mind
cannot go out of itself, cannot deal with anything but thought,
without ceasing to be mind; and matter must cease to be matter ere it
can lose its absolute externality, its nature as having _partes extra
partes_, and acquire the unity of mind. They are opposed as the
divisible and the indivisible, and there is no possible existence of
matter in thought except a representative existence. The ideal (or, as
Descartes calls it, objective) existence of matter _in_ thought and
the real (or, as Descartes calls it, formal) existence of matter _out
of_ thought are absolutely different and independent things.

Proof of existence of God.

It was, however, impossible for Descartes to be content with a
subjective idealism that confined all knowledge to the tautological
expression of self-consciousness "I am I," "What I perceive I perceive."
If the individual is to find in his self-consciousness the principle of
all knowledge, there must be something in it which transcends the
distinction of self and not self, which carries him beyond the limit of
his own individuality. What then is the point where the subjective
consciousness passes out into the objective, from which it seemed at
first absolutely excluded? Descartes answers that it is through the
connexion of the consciousness of self with the consciousness of God. It
is because we find God in our minds that we find anything else. The
proof of God's existence is therefore the hinge on which the whole
Cartesian philosophy turns, and it is necessary to examine the nature of
it somewhat closely.

Descartes, in the first place, tries to extract a criterion of truth out
of the _cogito, ergo sum_. Why am I assured of my own existence? It is
because the conception of existence is at once and immediately involved
in the consciousness of self. I can logically distinguish the two
elements, but I cannot separate them; whenever I clearly and distinctly
conceive the one, I am forced to think the other along with it. But this
gives me a rule for all judgments whatever, a principle which is
related to the _cogito, ergo sum_ as the formal to the material
principle of knowledge. Whatever we cannot separate from the clear and
distinct conception of anything, necessarily belongs to it in reality;
and on the other hand, whatever we can separate