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 truth 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