The purpose of these studies is to raise problems, not to solve them; to draw attention to a field of inquiry, rather than to survey it fully; and to provoke discussion rather than to serve as a systematic treatise. They are in three senses ‘essays’, being at the same time experimental incursions into the field with which they deal; assays or examinations of specimen concepts drawn rather arbitrarily from a larger class; and finally ballons d'essai, trial balloons designed to draw the fire of others. This being so, they may seem a little inconsequent. Some of the themes discussed will recur, certain central distinctions will be insisted on throughout, and for literary reasons I have avoided too many expressions of hesitancy and uncertainty, but nothing in what follows pretends to be final, and I shall have fulfilled my purpose if my results are found suggestive. If they are also found provoking, so much the better; in that case there is some hope that, out of the ensuing clash of opinions, the proper solutions of the problems here raised will become apparent.

What is the nature of these problems? In a sense they are logical problems. Yet it would perhaps be misleading to say that they were problems in logic, for the whole tradition of the subject would lead a reader to expect much that he will not find in these pages.