The real substance of the capital laid out in wages is labour itself, active, value-creating labour-power, living labour, which the capitalist exchanges for dead, materialised labour and embodies in his capital, by which means, and by which alone, the value in his hands turns into self-expanding value. But this power of self-expansion is not sold by the capitalist. It is always only a constituent part of his productive capital, the same as his instruments of labour; it is never a part of his commodity-capital, as for instance the finished product which he sells. In the process of production the instruments of labour, as components of the productive capital, are not opposed to labour-power as fixed capital any more than materials of labour and auxiliary substances are identified with it as circulating capital. Labour-power confronts both of them as a personal factor, while those are objective factors — speaking from the point of view of the labour-process. Both of them stand opposed to labour-power, as constant capital to variable capital — speaking from the point of view of the process of self-expansion of value. Or, if mention is to be made here of a material difference, so far as it affects the process of circulation, it is only this: It follows from the nature of value, which is nothing but materialised labour, and from the nature of active labour-power, which is nothing but labour in process of materialisation, that labour-power continually creates value and surplus-value during the time it functions; that what on the part of labour-power appears as motion, as a creation of value, appears on the part of its product in a state of rest, as created value. If the labour-power has performed its function, capital no longer consists of labour-power on the one side and means of production on the other. The capital-value that was invested in labour-power is now value which ( + surplus-value) was added to the product. In order to repeat the process, the product must be sold and new labour-power constantly bought with the proceeds and incorporated in the productive capital. This then gives to the part of capital invested in labour-power, and to that invested in material of labour, etc., the character of circulating capital as opposed to the capital remaining fixed in the instruments of labour.