The proposal that scepticism is possible in practice immediately gives rise to a further question: whether, and in what respect, scepticism is at all practically desirable. The host of issues associated with this question cannot be adequately discussed here. Before going on to discuss scepticism in the light of more or less purely epistemological considerations, we can at least pause to consider how scepticism stands with regard to an area in which the practical issues have already been formulated in comparatively precise terms, and the normative issues, as criteria for mental health, have been agreed on more or less, if only implicitly. What I propose to do here, therefore, is subject the radical sceptic to the test of currently accepted criteria of positive mental health.