But, putting aside this historical curiosity, it concerns us now to work at these ideas in order to advance in theoretical knowledge. How can historical materialism justify itself scientifically? This is the question I have proposed to myself, and to which the answer is given by the critical researches referred to at the beginning of this paragraph. Without returning to them I will give other examples, taken from the same source, that of the Marxian literature. How ought we to understand scientifically Marx's _neodialectic_? The final opinion expressed by Engels on the subject seems to be this: the dialect is the rhythm of the development of things, _i.e._ the inner law of things in their development. This rhythm is not determined _a priori_, and by metaphysical deduction, but is rather observed and gathered _a posteriori_, and only through the repeated observations and verifications that are made of it in various fields of reality, can it be presupposed that all facts develop through negations, and negations of negations.[44] Thus the dialect would be the discovery of a great natural law, less empty and formal than the so-called _law of evolution_ and it would have nothing in common with the old Hegelian dialect except the name, which would preserve for us an historical record of the way in which Marx arrived at it. But does this natural rhythm of development exist? This could only be stated from observation, to which indeed, Engels appealed in order to assert its existence. And what kind of a law is one which is revealed to us by observation? Can it ever be a law which governs things absolutely, or is it not one of those which are now called tendencies, or rather is it not merely a simple and limited generalisation? And this recognition of rhythm through negations of negations, it is not some rag of the old metaphysics, from which it may be well to free ourselves.[45] This is the investigation needed for the progress of science. In like manner should other statements of Marx and Engels be criticised. What for example shall we think of Engels' controversy with Dühring concerning the basis of history: whether this is _political force_ or _economic fact_? Will it not seem to us that this controversy can perhaps retain any value in face of Dühring's assertion that political fact is that _which is essential historically_, but in itself has not that general importance which it is proposed to ascribe to it? We may reflect for a moment that Engels' thesis: 'force protects (_schützt_) but does not cause (_verursacht_) usurpation,' might be directly inverted into another that: 'force _causes_ usurpation, but economic interest _protects_ it,' and this by the well known principle of the inter-dependence and competition of the social factors.