purchasing; a certain command over all the labour, or over all the produce of labour, which is then in the market. His fortune is greater or less, precisely in proportion to the extent of this power; or to the quantity either of other men’s labour, or, what is the same thing, of the produce of other men’s labour, which it enables him to purchase or command. The exchangeable value of everything must always be precisely equal to the extent of this power which it conveys to its owner.

But though labour be the real measure of the exchangeable value of all commodities, it is not that by which their value is commonly estimated.

It is of difficult to ascertain the proportion between two different quantities of labour. The time spent in two different sorts of work will not always