We cannot begin with complete doubt. We must begin with all the prejudices which we actually have when we enter upon the study of philosophy. These prejudices are not to be dispelled by a maxim, for they are things which it does not occur to us can be questioned. A person may, it is true, in the course of his studies, find reason to doubt what he began by believing; but in that case he doubts because he has a positive reason for it, and not on account of the Cartesian maxim. Let us not pretend to doubt in philosophy what we do not doubt in our hearts.
Charles Sanders Peirce (1839–1914), U.S. philosopher, logician. Originally published in Journal of Speculative Philosophy (1868). "Some Consequences of Four Incapacities," Collected Papers, vol. 5, para. 265, Harvard University Press (1934).
A comment on Descartes' maxim that we should begin by doubting everything.