The so-called materialist conception of history is not only very popular in certain quarters, it is also embodied in much of the practice of historians. Yet, in spite of the current interest in philosophies of history, it is not often that one finds it seriously and critically discussed by philosophers, or indeed by anybody. One reason for this is, no doubt, that though claiming to be scientific it is closely connected with a militant political and economic creed. But there are further and more honourable reasons, in the apparent comprehensiveness of the theory itself and in the lack of any comprehensive statement of it from its chief author. To some of its supporters it appears to be not merely a complete explanation of all history, past, present, and future, but also a psychology, a philosophy, and even a religion ; and yet Marx—who, if he did not originate it, is anyhow mainly responsible for its wide popularity—nowhere expounds it at any length, and for an adequate and authoritative discussion of it we must consult the correspondence of Engels, which is not gathered together in any one place. Thus, while opponents of materialism and determinism tend to reject or ignore a theory which is supposed to imply them both, opponents of Marxism are naturally unwilling to read through that author's very voluminous works for the purpose of reconstructing from his fragmentary remarks a theory that he nowhere fully states.