Are perceptual experiences reasons for perceptual beliefs? The act/content ambiguity of the term ‘belief’ carries over to this question. It is argued, following Popper, that experiences are reasons as well as causes for belief-acts, but not for belief-contents. This involves rejecting justificationism, the mistaken view that a reason for a belief-act must be a reason for its content. Popper's many critics presuppose justificationism, and so miss the main point of his solution to the problem of the empirical basis of science.