IN A would-be definite inquiry that I have been making into obsessional ill­ ness, I have been struck by the variety of problems and the difficulty of stating them. This would no doubt be true of any psychiatric topic as wide as obses­ sional illness, but here I found to my surprise that it would be harder to state the problems clearly than to present the alleged solutions offered in the litera­ ture. Some of these solutions deal with problems that are indefinite and in­ deed unsubstantial; others are global; they cover so wide a field that it is difficult to examine them without examining also the nature of man. It may well be that obsessional illness cannot be understood altogether without under­ standing the nature of man, or perhaps inquired into profitably without much bold speculation and the use of methods as yet unthought of or suspect; but one is reminded of Descartes’ rules-to doubt everything that is not clear, to avoid precipitancy, and to divide up every difficulty into as many parts as are possible and necessary for its better solution: also to proceed from the simplest and plainest facts. Obsessional illness has not usually been treated on such lines. I have tried in this paper to raise the difficult issues that seem to need clarification before an answer can well be sought, much less accepted.