I opened a book on philosophy today that my daughter gave me and I happened across the chapter on Descartes’ Principles of Philosophy.
This is how it began:
- The seeker after truth must, once in the course of his life, doubt everything, as far as is possible.
Since we began life as infants, and made various judgements concerning the things that can be perceived by the senses before we had the full use of our reason, there are many preconceived opinions that keep us from knowledge of the truth. It seems that the only way of freeing ourselves from these opinions is to make the effort, once in the course of our life, to doubt everything which we find to contain even the smallest suspicion of uncertainty.
- What is doubtful should even be considered as false.
Indeed, it will prove useful, once we have doubted these things, to consider them as false, so that our discovery of what is most certain and easy to know may be all the clearer.
- This doubt should not meanwhile be applied to ordinary life.
This doubt, while it continues, should be kept in check and employed solely in connection with the contemplation of the truth. As far as ordinary life is concerned, the chance for action would frequently pass us by if we waited until we could free ourselves from our doubts, and so we are often compelled to accept what is merely probable. From time to time we may even have to make a choice between two alternatives, even though it is not apparent that one of the two is more probable than the other.
- The reason for doubt concerning the things that can be perceived by the senses.
Given, then, that our efforts are directed solely to the search for truth, our initial doubts will be about the existence of the objects of sense-perceptions and imagination. The first reason for such doubts is that from time to time we have caught out the senses when they were in error, and it is prudent never to place too much trust in those who have deceived us even once. The second reason is that in our sleep we regularly seem to have sensory perception of, or to imagine, countless things which do not exist anywhere; and if our doubts are on the scale just outlined, there seem to be no marks by means of which we can with certainty distinguish being asleep from being awake.