it, the fact that what he offers as the truth is strange ought 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 without being in my mind must be in some other mind.
In order to understand his argument, it is necessary to understand his use of the word “idea.” He gives the name “idea” to anything which is immediately known, as, for example, sense-data are known. Thus a particular colour which we see is an idea; so is a voice which we hear, and so on. But the term is not wholly confined to sense-data. There will also be things remembered or imagined, for with such things also we have immediate acquaintance at the moment of remembering or imagining. All such immediate data he calls “ideas.”
He then proceeds to consider common objects, such as a tree, for instance. He shows that all we know immediately when we “perceive” the tree consists of ideas in his sense of the word, and he argues that there is not the slightest ground for supposing that there is anything real about