Page:Russell - The Problems of Philosophy, 1912.djvu/64

60 not to be made a ground of objection to his opinion.

The grounds on which idealism is advocated are generally grounds derived from the theory of knowledge, that is to say, from a discussion of the conditions which things must satisfy in order that we may be able to know them. The first serious attempt to establish idealism on such grounds was that of Bishop Berkeley. He proved first, by arguments which were largely valid, that our sense-data cannot be supposed to have an existence independent of us, but must be, in part at least, "in" the mind, in the sense that their existence would not continue if there were no seeing or hearing or touching or smelling or tasting. So far, his contention was almost certainly valid, even if some of his arguments were not so. But he went on to argue that sense-data were the only things of whose existence our perceptions could assure us, and that to be known is to be "in" a mind, and therefore to be mental. Hence he concluded that nothing can ever be known except what is in some mind, and that whatever is known