His meaning in this respect is illustrated with pictorial vividness by his teaching with regard to the form in which the measure of exchange should embody itself.

This, he said, ought not to be gold or silver, but "labour-certificates," which would indicate that whoever possessed them had laboured for so many hours in producing no matter what, and which would purchase anything else, or any quantity of anything else, representing an equal expenditure of labour of any other kind.
Having thus settled, as it seemed to him beyond dispute, that manual labour, estimated in terms of time, is the sole source and measure of economic values or of wealth, Marx goes on to point out that, by the improvement of industrial methods, labour in the modern world has been growing more and more productive, so that each labour-hour results in an increased yield of commodities.

Thus a man who a couple of centuries ago could have only just kept himself alive by the products of his entire labour-day, can now keep himself alive by the products of half or a quarter of it.

The products of the remainder of his labour-day are what Marx called a "surplus value," meaning by this phrase all that output of wealth which is beyond what is practically necessary to keep the labourer alive.

But what, he asks, becomes of this surplus?
Does it go to the labourers who have produced it?
No, he replies.