ought, if my thought is my being, and the converse, if in me thought
and existence are identical, then I am a being whose essence consists in
thinking. I am a spirit, an ego, a rational soul. My existence follows only
from my thinking, not from any chance action. _Ambulo ergo sum_ would not
be valid, but _mihi videor_ or _puto me ambulare, ergo sum_. If I believe
I am walking, I may undoubtedly be deceived concerning the outward action
(as, for instance, in dreams), but never concerning my inward belief.
_Cogitatio_ includes all the conscious activities of the mind, volition,
emotion, and sensation, as well as representation and cognition; they are
all _modi cogitandi_. The existence of the mind is therefore the most
certain of all things. We know the soul better than the body. It is for
the present the only certainty, and every other is dependent on this, the
highest of all.

What, then, is the peculiarity of this first and most certain knowledge
which renders it self-evident and independent of all proof, which makes
us absolutely unable to doubt it? Its entire clearness and distinctness.
Accordingly, I may conclude that everything which I perceive as clearly and
distinctly as the _cogito ergo sum_ is also true, and I reach this general
rule, _omne est verum, quod clare et distincte percipio_. So far, then, we
have gained three things: a challenge; to be inscribed over the portals
of certified knowledge, _de omnibus dubitandum_; a basal truth, _sum
cogitans_; a criterion of truth, _clara et distinct a perceptio_.

The doubt of Descartes is not the expression of a resigned spirit which
renounces the unattainable; it is precept, not doctrine, the starting point
of philosophy, not its conclusion, a methodological instrument in the hand
of a strong and confident longing for truth, which makes use of doubt to
find the indubitable. It is not aimed at the possibility of attaining
knowledge, but at the opinion that it has already been attained, at the
credulity of the age, at its excessive tendency toward historical and
poly-historical study, which confuses the acquisition and handing down of
information with knowledge of the truth. That knowledge alone is certain
which is self-attained and self-tested--and this cannot be learned
or handed down; it can only be rediscovered through examination and
experience. Instead of taking one's own unsupported conjectures or the
opinions of others as a guide, the secret of the search for tr