VARIETIES OF IDEALISM
By the second half of the nineteenth century, it had become a hazardous occupation for a philosopher to talk about our knowledge of the external world. Early in modern thought, mind and matter had become separated by the widely-accepted division of all substances into two classes, one called, broadly, thinking substance, mind, soul or spirit; the other called extended substance, body or matter. Important as it may have been to make this distinction in its day, it soon became evident that the making of it led to the posing of a significant problem of knowledge, which has become the central problem of modern philosophy. For if thinking substance and extended substance are different in kind, the question is bound to be asked how thinking substance, which is mind, can know extended substance, which is matter. Some of the proposed resolutions of the difficulty accepted the dualism of mind and matter, consciousness and the external world, at its face value. They proposed the simple answer that it is the nature of consciousness to know the external world, the nature of the external world to be known by consciousness. This naive answer does not, of course, solve anything; it leaves the problem where it was but provides a statement on the basis of which speculation can be carried on despite the unsolved problem at its base.
Other solutions took reductive forms; without breaking down the mind-matter dualism as it was originally stated, these thinkers evaded its implications for human knowledge either by the assertion that what we call mind is not really mind or that what we refer to as matter is not really matter. In the first case, mind is thought to be material action of one sort or another, and thus mind is reduced