Russell's chs. 3 and 4 address the second of the questions associated with idealism (see ¶¶ 1.9-1.11), the nature of external objects. The first presents his view and the second considers arguments for idealism. If the key philosopher in the background of chs. 1 and 2 was Descartes, the one Russell has in mind here is Berkeley. In ch. 3, Russell supports Berkeley's extension of the views of Descartes and Locke concerning "secondary qualities" (i.e., color, sound, etc.) to Locke's "primary qualities" (i.e., number, size, shape, etc.); and, in ch. 4, it is Berkeley's idealism that Russell is concerned to reject.

• The heart of ch. 3 is the distinction between real and apparent space and the relation between the two, but also think about Russell's positive account of what we can know about external objects (see ¶¶ 3.9-3.12).

• In working through ch. 4, think about the two arguments for idealism that Russell considers (beginning with ¶¶ 4.4 and 4.12, respectively) and his criticisms of them. But also think about the distinctions he makes in criticizing these arguments. The act/object distinction used in discussing the first has medieval origins but you've encountered it already in Russell as the distinction between sensations and sense data in ch. 1 (see ¶ 1.10). The second set of distinctions, one between knowledge of things and knowledge of facts and another between knowledge of things "by acquaintance" and knowledge of them "by description," counts as one of the two or three most important ideas in Russell's book.