The great instrument of production is labour. There is no commodity nor implement used to assist manual exertions in which it does not enter as a condition of supply, and very few in which it does not enter very largely. If in the production of commodities and of the implements which assist in this production, no other ingredient were required than labour, and the interval between the exertion of the labour and its remuneration in the completed commodity were so inconsiderable that it might be entirely disregarded, it is certain that, as the same quantity of labour would have a constant tendency to produce commodities in the same relative proportion to each other, and to the demand for them, they would be found on an average to exchange with each other according to the quantity of labour which had been employed to obtain them. Thus if ten mackerel were, on an average, obtained by the same quantity of labour as two soals, it would be necessary, in order to continue the supply of both in the market, that the value of a soal should be five times as great in the power of purchasing similar commodities, as the value of a mackerel; because if it were less, none would apply themselves to the catching of soals; and though it is quite certain that at any given period the relative value of soals and mackerel would be exclusively determined by the state of the demand and supply of each; and that they would, in consequence, often vary very considerably; yet it is as certain, that on the supposition of the hypothesis being correct, and that they both continued to be brought to market, each would on an average be supplied in such a quantity, compared with the demand for it, that a soal would ordinarily exchange for five mackerel, and the different quantities of labour required to produce them would, in this case, be a correct measure both of their natural and relative value in exchange.