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.

The Gospel in a Pluralist Society. Eerdmans, 1989 (reprinted 2002),19.