Of your most Serene Highness, The most devoted servant,

DESCARTES.


OF THE PRINCIPLES OF HUMAN KNOWLEDGE.

I. THAT in order to seek truth, it is necessary once in the course
of our life, to doubt, as far as possible, of all things.

As we were at one time children, and as we formed various judgments
regarding the objects presented to our senses, when as yet we had
not the entire use of our reason, numerous prejudices stand in the
way of our arriving at the knowledge of truth; and of these it seems
impossible for us to rid ourselves, unless we undertake, once in our
lifetime, to doubt of all those things in which we may discover even
the smallest suspicion of uncertainty.

II. That we ought also to consider as false all that is doubtful.

Moreover, it will be useful likewise to esteem as false the things
of which we shall be able to doubt, that we may with greater
clearness discover what possesses most certainty and is the easiest
to know.

III. That we ought not meanwhile to make use of doubt in the
conduct of life.

In the meantime, it is to be observed that we are to avail ourselves