This is an excerpt from Bertrand Russell’s “Problems of Philosophy”:
Descartes (1596-1650), the founder of modern philosophy, invented a method which may still be used with profit–the method of systematic doubt. He determined that he would believe nothing which he did not see quite clearly and distinctly to be true. Whatever he could bring himself to doubt, he would doubt, until he saw reason for not doubting it.
The second sentence…
… could be simplified as follows: “He determined to believe nothing which was not true.” which is nonsense – if you do away with the double negative you have “He determined to believe the truth.”
A proposition is either true or false. If something is false or true, how can one “determine” to doubt or believe it?
The third sentence:
You cannot “bring yourself” to doubt something any more than you can “bring yourself “to believe something. Doubt arises when what is proposed does not correspond with what is known. Doubt is the clash between what is known to be true and that which is claimed to be true. Doubt (or belief) cannot be avoided.
If I touch a surface and feel that it is cold, I cannot doubt that it is cold, because I already know what a cold surface feels like.
If you say to me that you possess a red car, I can only doubt that claim if what you possess is not what I believe to be a car, or I agree that what you have is a car, but its colour does not correspond to what I believe to be the colour red.
If someone draws a figure on paper and claims it to be a square, I can only doubt if it’s a square if it does not correspond to what I know a square should be. Whether I judge the figure by my eye or by a ruler I must know what a square is to be able to judge if the figure corresponds with my knowledge.
You cannot doubt if a square is a square unless you know what a square is!
Doubt arises when what is claimed to be true does not correspond with what we know, or believe, to be true.
You cannot doubt something unless you already have a truth by which to judge it.
We live in a fortress of certainty, we can do no other – the truth of our perceptions of the world are vital to our survival. This fortress maintains its structure until the beliefs that it is built from are proved to be false. And such proofs do not diminish and weaken but enlarge and strengthen the fortress.
There must be certainty before there can be doubt. To doubt everything would be to know all.