uth is to
become independent and of age, to think for one's self; and the only remedy
against the dangers of self-deception and the ease of repetition is to be
found in doubting everything hitherto considered true. This is the meaning
of the Cartesian doubt, which is more comprehensive and more thorough
than the Baconian. Descartes disputed only the certitude of the knowledge
previously attained, not the possibility of knowledge--for of the latter no
man is more firmly convinced than he. He is a rationalist, not a skeptic.
The intellect is assured against error just as soon as, freed from
hindrances, it remains true to itself, as it puts forth all its powers and
lets nothing pass for truth which is not clearly and distinctly known.
Descartes demands the same thing for the human understanding as Rousseau at
a later period for the heart: a return to uncorrupted nature. This faith in
the unartificial, the original, the natural, this radical and naturalistic
tendency is characteristically French. The purification of the mind, its
deliverance from the rubbish of scholastic learning, from the pressure of
authority, and from inert acceptance of the thinking of others--this is
all. Descartes finds the clearest proof of the mind's capacity for truth in
mathematics, whose trustworthiness he never seriously questioned, but only
hypothetically, in order to exhibit the still higher certainty of the "I
think, therefore I am." He wants to give philosophy the stable character
which had so impressed him in mathematics when he was a boy, and recommends
her, therefore, not merely the evidence of mathematics as a general
example, but the mathematical method for definite imitation. Metaphysics,
like mathematics, must derive its conclusions by deduction from
self-evident principles. Thus the geometrical method begins its rule in
philosophy, a rule not always attended with beneficial results.

With this criterion of truth Descartes advances to the consideration of
ideas. He distinguishes volition and judgment from ideas in the narrow
sense (_imagines_), and divides the latter, according to their origin, into
three classes: _ideae innatae, adventitiae, a me ipso factae_, considering
the second class, the "adventitious" ideas, the most numerous, but the
first, the "innate" ideas, the most important. No idea is higher or clearer
than the idea of God or the most perfect being. Whence comes this idea?
That every idea must have a cause, follows from