But even this delusion could not long prevail. Ricardo was the first of the moderns who perceived that wealth did not increase in proportion to industrial capacity, even when production and trade were, as Smith demanded, freed from State interference and injury. He hit upon the expedient of finding the cause of this incongruity in the nature of labour itself. Since labour is the only source of value, he said, it cannot increase value. A thing is worth as much as the quantity of labour put into it; consequently, when with increasing productiveness of labour the amount of labour necessary to the production of a thing is diminished, then the value of that thing diminishes also. Hence no increase in the productiveness of labour can increase the total sum of values. This, however, is a fundamental mistake, for what depends upon the amount of labour is merely the _relative_ value of things--the exchange relation in which they stand to other things. This is so self-evident that Ricardo himself cannot avoid expressly stating that he is speaking of merely the 'relative' value of things; nevertheless, this relative value--which, strictly speaking, is nothing but a value relation, the relation of values--is treated by him as if it were absolute value. And yet Ricardo's error is a not less important step in the evolution of doctrine than those of his previously mentioned predecessors. It signifies the revival of the original problem of political economy, which had been lost sight of since Adam Smith; and Ricardo's follower, Marx, is in a certain sense right when, with bitter scorn, he denounces as 'vulgar economists' those who, persistently clinging to Smith's optimism, see in the _productiveness_ of labour the measure of the increase of _actual_ wealth. For all that was brought against Ricardo by his opponents was known by him as well as or better than by them; only he knew what had escaped their notice, or what they saw no obligation to take note of in their theory--namely, that the actual facts directly contradicted the doctrine. It by no means escaped Ricardo that his attempted reconciliation of the theory with the great problem of economics was absurd; and Marx has most clearly shown the absurdity of it. The latter speaks of the alleged dependence of value, not upon the productiveness of labour, but upon the effort put forth by the labourer, as the 'fetishism' of industry; this relation, being unnatural, contrary to the nature of things, ought therefore--and this, again, is Marx's contribution to the progress of the science--to be referred back to an unnatural ultimate cause residing, not in the nature of things, but in human arrangements. And in looking for this ultimate cause, he, like his great predecessors, comes extremely near to the truth, but, after all, glides past without seeing it.