“The value of labour-power itself is not fixed: it changes with the prices of necessities, with variations in the cost of training, and with physiological differences. Its relations to surplus value extracted also vary. When other factors (hours, intensity of labour) are constant and productiveness varies, the surplus value and labour-power vary in opposite directions. The value of labour-power goes down and surplus value goes up when productiveness increases. When the intensity of labour varies, the value of labour-power increases as it is used up more quickly in a given time. When the length of the working day is varied, the value of labour falls relatively to the surplus value produced. And so on. The formula for discovering surplus value given in the older economists is thus incorrect. Marx’s formula briefly expressed is: the excess of Unpaid over Paid Labour. Capital is thus essentially the command of unpaid labour. (…) The laws of time wages, as has already been suggested, correspond to the laws governing the relative value of labour-power and of surplus value. There are, however, certain peculiarities to be noted: for example, when a man is paid by very short time intervals he may never even produce his own subsistence. If he is employed by the hour, for instance, he may be employed in the total so few hours that he never can produce his own necessities, though surplus value has been extracted from him” [Raymond W. Postgate, Karl Marx, 1933]