The doctrine of idealism holds that "whatever can be known to exist, must be in some sense mental." The character of this doctrine opposes our common sense view that ordinary, physical objects like the table or the sun are made up of something very different from what we call "mind" or our "thoughts." We think of the external world as independent and holding physical things made of matter. Compared with the common sense view, idealism is plainly harder to believe. In the last chapter, Russell claimed that the way in which physical objects exist differs radically from our notion of sense-data; although, they do share a correspondence. Neither this relation nor common sense justified the possibility of a direct way of knowing the real nature of the outside world. The rejection of idealism on the basis that it runs counter to common sense thus seems premature.
This chapter reviews the grounds upon which the notion of idealism is built. Russell begins with arguments made by Bishop Berkeley. Berkeley couched his philosophy in the edifice of a theory of knowledge. He argued that the objects of sensation, our sense-data, must depend on us in the sense that if we stopped hearing or tasting or seeing or perceiving, then the sense-data could not continue to exist. It must exist, in some part, in a mind. Russell allows that Berkeley's reasoning thus far is "valid." However, further extrapolations are less valid. Berkeley continued that the only things of which our perceptions could make us sure of their existence were sense-data. Since sense-data existed in the mind, then all things that could be known existed in a mind. Reality was a product of some mind, and any "thing" not in some other mind does not exist.
Berkeley called the pieces of sense-data, or things that could be immediately known, "ideas." Memories and things imagined could also be immediately known by virtue of the way the mind works and were also called ideas. Something like a tree exists, according to Berkeley, because someone perceives it. What is real about a tree exists in its perception, an idea from which the famous philosophic idiom: esse is percipi derives; the tree's being is in its being perceived. But what if no human perceives the tree? Berkeley admitted belief in an external world independent of humans. His philosophy held that the world and everything in it was an idea in the mind of God. What we call a real thing is the continuing "physical" object or permanent idea in God's mind. Our minds participate in God's perceptions, and thus different people's differing perceptions of the same object are variable but similar because each is of a piece with the same thing. Nothing could possibly exist or be known except these "ideas."
Russell responds to Berkeley's idealism with a discussion of the word "idea." Russell claims that Berkeley generates a use of the word that makes it easier to believe the arguments advanced for idealism. Since we think of ideas as mental things anyway, when we are told that a tree is an idea, an easy application of the word "idea" places the tree in our minds. Russell suggests that the notion of something being "in the mind" is hard to understand. We speak of bearing some concept or some person "in mind," meaning that the thought of it or him is in our mind, not the thing itself. And thus, "when Berkeley says that the tree must be in our minds if we can know it, all that he really has a right to say is that a thought of the tree must be in our minds." Russell says that Berkeley's meaning is in gross confusion. He attempts to unravel the sense in which Berkeley engages sense-data and the physical world. Berkeley treated the notion of sense-data as something subjective, depending on us for its existence. He made this observation, then sought to prove that anything that "can be immediately known" is in the mind and only in the mind. Russell points out that the observation about the dependence of sense-data does not lead to the proof Berkeley seeks. What he would need to prove is "that by being known, things are shown to be mental."
Russell continues to consider the nature of ideas, in order to analyze the grounds of Berkeley's argument. Berkeley refers to two different things using the same word, "idea." One is the thing of which we become aware, like the color of Russell's table, and the other is the actual act of apprehension. While the latter act seems obviously mental, the former "thing" does not seem so at all. Berkeley, Russell argues, produces the effect of natural agreement between these two senses of "idea." We agree that the apprehending takes place in the mind, and by this we soon arrive at an understanding in the other sense, that things that we apprehend are ideas and are also in the mind. Russell calls this sleight of reasoning an "unconscious equivocation." We find ourselves at the end believing that what we can apprehend has been in our minds, the "ultimate fallacy" of Berkeley's argument.
Russell has made a distinction between act and object, using the sense of "idea." He returns to it because he claims that our entire system of acquiring knowledge is involved with it. Learning and becoming acquainted with something involves a relation between a mind and something, anything, other than that mind. If, with Berkeley, we agree that things that can be known exist in the mind alone, then we instantly limit man's capacity to gain knowledge. To say that what we know is "in the mind" as if we mean "before the mind" is to speak a tautology. Yet, this leads to the contradictory conclusion that what may be before the mind may not be in the mind as it may not be mental. The nature of knowledge itself refutes Berkeley's argument. Russell dismisses Berkeley's argument for idealism.