ly exists cannot be questioned, even by the most zealous advocates of that law. Just as little can it be questioned that, under certain circumstances, the consideration of the postponement may have even a greater influence on the value of goods than the consideration of the amount of labour-costs. I may remind the reader, for example, of the value of an old wine that has been stored up for scores of years, or of a hundred years old tree in the forest.
But on that exception hangs a tale. It does not require any great penetration to see that the principal feature of natural interest on capital is really involved in it. For when, on the division of the value, those goods that require for their production an advance of foregoing labour show a surplus of exchange value, it is just this surplus that remains in the hands of the capitalist-undertaker as profit. If this difference of value did not exist natural interest on capital would not exist either. This difference of value makes it possible, contains it, is identical with it.
Nothing is more easily demonstrated than this, if any proof is wanted of so obvious a fact. Supposing each of three goods requires for its making a year's labour, but a different length of time over which the labour is advanced. The first good requires only one year's advance of the year's labour; the second a ten years' advance; the third a twenty years' advance. Under these circumstances the exchange value of the first good will, and must be, sufficient to cover the wages of a year's labour, and, beyond that, one year's interest on the advanced labour. It is perfectly clear that the same exchange value cannot be sufficient to cover the wages of a year's labour, and a ten or twenty years' interest on the ten or twenty years' advance of labour as well. That interest can only be covered if and because the exchange value of the second and third good is correspondingly higher than that of the first good, although all three have cost an equal amount of labour. The difference of exchange value is clearly the source from which the ten and twenty years' interest flows, and the only source from which it can flow.
Thus this exception to the law of labour-value is nothing less than the chief feature in natural interest on capital. Any one who would explain natural interest must, in the first place, explain this; without an explanation of the exception here can be no explanation of the problem of interest. Now if, notwithstanding, in treatises on interest this exception is ignored, not to say denied, it is as gross a blunder as could well be conceived. When Rodbertus ignores the exception, it means nothing else than ignoring the chief part of what he ought to have explained.
Nor can one excuse Rodbertus's blunder by saying that he did not intend to lay down a rule which should hold in actual life, but only a hypothetical assumption by which he might carry through his abstract inquiries more easily and more correctly. It is true that Rodbertus, in some passages of his writings, does clothe the proposition, that the value of all goods is determined by their labour costs, in the form of a simple hypothesis.*46 But, firstly, there are many passages where Rodbertus expresses his conviction that his principle of value also holds in actual economic life.*47 And, secondly, a man may not assume anything that he likes, even as a simple hypothesis. That is to say, even in a purely hypothetical assumption, one may omit only such circumstances of actual fact as are irrelevant to the question under examination. But what is to be said for a theoretical inquiry into interest which at the critical point leaves out the existence of the most important feature; which gets rid of the principal part of what it had to explain with a "let us assume"?
On one point it may be admitted that Rodbertus is right: if we wish to discover a principle like that of land-rent or interest, we must "not let value dance up and down";*48 we must assume the validity of a fixed law of value. But is it not also a fixed law of value that goods which require