Marx has no sooner reduced value to the single uniform element of time of labour, and excluded from its constitution all considerations of utility and the state of the market, than he reintroduces those considerations under a disguised form. In the first place, if a day's labour of given length always produces the same value, it is obvious to ask whether then an indolent and unskilful tailor who takes a week to make a coat has produced as much value as the more expert hand who turns out six in this time, or, with the help of a machine, perhaps twenty? Marx answers, Certainly not, for the time of labour which determines value is not the time actually taken, but the time required in existing social conditions to produce that particular kind of commodity--the time taken by labour of average efficiency, using the means which the age affords--in short, what he calls the socially necessary time of labour. Value is an immanent relation to socially necessary time of labour. Marx's standard is thus, after all, not one of quantity of labour pure and simple; it takes into account, besides, the average productive power of labour in different branches of industry. "The value of a commodity," says he, "changes directly as the quantity, and inversely as the productive power, of the labour which realizes itself in that commodity." Before we know the value of a commodity we must therefore know not only the quantity of labour that has gone into it, but the productive power of that labour. We gather the quantity from the duration of exertion, but how is average productive power to be ascertained? By simply ascertaining the total product of all the labour engaged in a particular trade, and then striking the average for each labourer. Diamonds occur rarely in the crust of the earth, and therefore many seekers spend days and weeks without finding one. Hits and misses must be taken together; the productive power of the diamond seeker is low; or, in other words, the time of labour socially necessary to procure a diamond is high, and its value corresponds. In a good year the same labour will produce twice as much wheat as in a bad; its productive power is greater; the time socially necessary to produce wheat is less, and the price of the bushel falls. The value of a commodity is therefore influenced by its comparative abundance, whether that be due to nature, or to machinery, or to personal skill.