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 the tree except what is perceived. Its being, he says, consists in being perceived: in the Latin of the school-men its "esse" is "percipi." He fully admits
