The rent doctrine gives one essential datum. A clear comprehension of rent is, as he was persuaded, 'of the utmost importance to political economy.'[303] The importance is that it enables him to separate one of the primary sources of revenue from the others. It is as though, in the familiar illustration, we were considering the conditions of equilibrium of a fluid; and we now see that one part may be considered as a mere overflow, resulting from (not determining) the other conditions. The primary assumption in the case of the market is the level of price. When we clearly distinguish rent on one side from profits and wages on the other, we see that we may also assume a level of profits. There cannot, as Ricardo constantly says, 'be two rates of profit,' that is, at the same time and in the same country. But so long as rent was lumped with other sources of revenue it was impossible to see, what Malthus and West had now made clear, that in agriculture, as in manufactures, the profits of the producer must conform to the principle. Given their theory, it follows that the power of land to yield a great revenue does not imply a varying rate of profit or a special bounty of nature bestowed upon agriculture. It means simply that, since the corn from the good and bad land sells at the same price, there is a surplus on the good. But as that surplus constitutes rent, the farmer's rate of profit will still be uniform. Thus we have got rid of one complication, and we are left with a comparatively simple issue. We have to consider the problem, What determines the distribution as between the capitalist and the labourer? That is the vital question for Ricardo.