it. By applying this method he gradually became convinced that the
only existence of which he could be _quite_ certain was his own. He
imagined a deceitful demon, who presented unreal things to his senses
in a perpetual phantasmagoria; it might be very improbable that such a
demon existed, but still it was possible, and therefore doubt
concerning things perceived by the senses was possible.

But doubt concerning his own existence was not possible, for if he did
not exist, no demon could deceive him. If he doubted, he must exist;
if he had any experiences whatever, he must exist. Thus his own
existence was an absolute certainty to him. 'I think, therefore I
am,' he said (_Cogito, ergo sum_); and on the basis of this certainty
he set to work to build up again the world of knowledge which his
doubt had laid in ruins. By inventing the method of doubt, and by
showing that subjective things are the most certain, Descartes
performed a great service to philosophy, and one which makes him still
useful to all students of the subject.

But some care is needed in using Descartes' argument. 'I think,
therefore I am' says rather more than is strictly certain. It might
seem as though we were quite sure of being the same person to-day as
we were yesterday, and this is no doubt true in some sense. But the
real Self is as hard to arrive at as the real table, and does not seem
to have that absolute, convincing certainty that belongs to particular
experiences. When I look at my table and see a certain brown colour,
what is quite certain at once is not '_I_ am seeing a brown colour',
but rather, 'a brown colour is being seen'. This of course involves
something (or somebody) which (or who) sees the brown colour; but it
does not of itself involve that more or less permanent person whom we
call 'I'. So far as immediate certainty goes, it might be that the