In the past 300 years philosophy has been preoccupied with the problem of knowledge. Since Descartes, traditional, prescientific culture has eroded under the sceptical attack and has been replaced by a new culture characterized by an unprecedented growth in scientific knowledge and its powerful models of explanation. Gellner seeks to understand "the differences between its two shores, the nature of the reasons and causes which explain or justify our firm location on one side of it." His concern is with relativism, not only how to state the problem, but also how to find norms independent of any belief system which would codify or justify one system rather than another one. What this comes to is the legitimation of belief, the search for and use of final norms which are legitimate prior to any specific information about the world and serve as the final court of appeal for all study. This he claims is the central problem of the theory of knowledge.

Gellner begins by "decoding" the history of philosophy since Descartes. Rather than reading the history of modern philosophy as saying what knowledge is really like, he reads it as a search for alternative theories of cognitive legitimacy. Two types of "anchor points" are dominant: selector theories and re-endorsement theories. Re-endorsement theories approve existing beliefs and are of three types: relativism, evolutionism, and negative re-endorsement. These are characteristic of the nineteenth century. On the other hand, selector theories seek to justify beliefs and are found in the seventeenth, eighteenth and twentieth centuries. They are empiricism, materialism, and logical form or skeletalism. Gellner contends that the re-endorsement theories will not help us solve the central problem of epistemology. Furthermore, little is to be learned from the skeletal theories of such thinkers as the early Wittgenstein, Russell, and Quine. However, the selector theories, empiricism and materialism, while inadequate, deserve careful consideration. Empiricism, the ghost or bundle theory, attempts to justify knowledge by appeal to experience. Materialism, or the machine theory, attempts to justify knowledge by appeal to a specification of publicly reproducible structure. The discussion of the differences, similarities, and cohabitation of the ghost and the machine selectors is a lengthy and penetrating study of some of the most important features of modern scientific, industrial culture.

Gellner continues his attempt to understand the cognitive style of modern industrial civilisation by characterizing prescientific thought. What distinguishes modernity or science from the savage mind? One of the most important differences is that in the traditional belief system discovery and endorsement of truth is an event having to do with the social and moral world, while in the modern scientific age discovery and endorsement of truth is extra-territorial. This means truth is autonomous from social, moral, political obligations, and decencies of a society. The implication of extra-territoriality is relativism in the social, moral dimension of society. The issue remains, to what final court is one to appeal in the face of the relativism of modern civilization? While he gives no final answer to this issue, Gellner discusses the major elements which must be part of the position we must take.

This is a fresh interdisciplinary approach to the foundations problems written with wit and verve. Gellner's treatment of diverse thinkers such as Kant, Popper, Kuhn, Russell, Wittgenstien, Ryle, and Weber is fresh and insightful. Yet he overlooks the significant work of Frankfort and Jacobson in his study of the "savage" mind. Also, little attention is given to teleological explanation.