The same difficulty certainly occurs in regard to the writings of numerous thinkers; but it is especially great in regard to those of Marx. And the interpreter must proceed with caution: he must do his work bit by bit, book by book, statement by statement, connecting indeed these various indications one with another, but taking account of differences of time, of actual circumstances, of fleeting impressions, of mental and literary habits; and he must submit to acknowledge ambiguities and incompleteness where either exists, resisting the temptation to confirm and complete by his own judgment. It may be allowed for instance, as it appears to me for various reasons, that the way in which historical materialism is stated above is the same as that in which Marx and Engels understood it in their inmost thoughts; or at least that which they would have agreed to as correct if they had had more time available for such labours of scientific elaboration, and if criticism had reached them less tardily. And all this is of importance up to a certain point, for the interpreter and historian of ideas; since for the history of science, Marx and Engels are neither more nor less than they appear in their books and works; real, and not hypothetical or possible persons.[41] But even for science itself, apart from the history of it, the hypothetical or possible Marx and Engels have their value. What concerns us theoretically is to understand the various possible ways of interpreting the problems proposed and the solutions thought out by Marx and Engels, and to select from the latter by criticism those which appear theoretically true and welcome. What was Marx's intellectual standpoint with reference to the Hegelian philosophy of history? In what consisted the criticism which he gave of it? Is the purport of this criticism always the same for instance in the article published in the _Deutsch-französische Jahrbücher_, for 1844, in the _Heilige Familie_ of 1845, in the _Misère de la philosophie_ of 1847, in the appendix to _Das Komnunistische Manifest_ of 1848, in the preface to the _Zur Kritik_ of 1859, and in the preface to the 2nd edition of _Das Kapital_ of 1873? Is it again in Engels' works in the _Antidühring_, in the article on _Feuerbach_, etc.? Did Marx ever really think of substituting, as some have believed, _Matter_ or material fact for the Hegelian _Idea_? And what connection was there in his mind between the concepts _material_ and _economic_? Again, can the explanation given by him, of his position with regard to Hegel: 'the ideas determined by facts and not the facts by the ideas,' be called an inversion of Hegel's view, or is it not rather the inversion of that of the ideologists and doctrinaires?[42] These are some of the questions pertaining to the _history of ideas_, which will be answered some time or other: perhaps at present the time has not yet arrived to write the history of ideas which are still in the process of development.[43]