"There is a need for what [Michael] Polanyi calls the critique of doubt. When we undertake to doubt any statement, we do so on the basis of beliefs which--in the act of doubting--we do not doubt. I can only doubt the truth of a statement on the ground of other things--usually a great many things--which I believe to be true. It is impossible at the same time to doubt both the statement, and the beliefs on the basis of which the statement is doubted."