Exempt from Doubt
We know, with the same certainty with which we believe any mathematical proposition, how the letters A and B are pronounced, what the colour of human blood is called, that other human beings have blood and call it "blood". That is to say, the questions that we raise and our doubts depend on the fact that some propositions are exempt from doubt, are as it were like hinges on which those things turn. That is to say, it belongs to the logic of our scientific investigations that certain things are in deed not doubted. But it isn't that the situation is like this: We just can't investigate everything, and for that reason we are forced to rest content with assumption. If I want the door to turn the hinges must stay put. My life consists in being content to accept many things. --Ludwig Wittgenstein, On Certainty 340-44