The Selections from the Principles of Philosophy

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.

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.