Doubts about Doubt
One cannot make experiments if there are not some things that one does not doubt. But that does not mean that one takes certain presuppositions on trust. When I write a letter and post it, I take it for granted that it will arrive--I expect this. If I make an experiment I do not doubt the existence of the apparatus before my eyes. I have plenty of doubt, but not that . If I do a calculation I believe, without any doubts, that the figures on the paper aren't switching of their own accord, and I also trust my memory the whole time, and trust it without any reservation. The certainty here is the same as that of my never having been on the moon. --Ludwig Wittgenstein, On Certainty 337