of this general doubt only while engaged in the contemplation of
truth. For, as far as concerns the conduct of life, we are very
frequently obliged to follow opinions merely probable, or even
sometimes, though of two courses of action we may not perceive more
probability in the one than in the other, to choose one or other,
seeing the opportunity of acting would not unfrequently pass away
before we could free ourselves from our doubts.

IV. Why we may doubt of sensible things.

Accordingly, since we now only design to apply ourselves to the
investigation of truth, we will doubt, first, whether of all the
things that have ever fallen under our senses, or which we have ever
imagined, any one really exist; in the first place, because we know
by experience that the senses sometimes err, and it would be
imprudent to trust too much to what has even once deceived us;
secondly, because in dreams we perpetually seem to perceive or
imagine innumerable objects which have no existence. And to one who
has thus resolved upon a general doubt, there appear no marks by
which he can with certainty distinguish sleep from the waking state.

V. Why we may also doubt of mathematical demonstrations.

We will also doubt of the other things we have before held as most
certain, even of the demonstrations of mathematics, and of their
principles which we have hitherto deemed self-evident; in the first
place, because we have sometimes seen men fall into error in such
matters, and admit as absolutely certain and self evident what to us
appeared false, but chiefly because we have learnt that God who
created us is all-powerful; for we do not yet know whether perhaps