 The view that knowledge must be of what is independent is the second prong of Prichard’s persistent attacks on the notion of a sense-datum (and thereby on Russell, Moore and Price). A sense-datum, he says, is supposed to be something of whose existence we know in every case of perception. But sense-data are not supposed to have an independent existence, and therefore the very conception of a sense-datum involves a contradiction. As Prichard came to see it, the basic mistake made by defenders of a sense-datum theory is that of supposing that perceiving is a form of knowing; all the other errors flow from this (Prichard 1938; see also his 1909, p. 99).