In the controversy concerning the populousness of ancient and modern nations, could it be clearly ascertained that the average produce of the countries in question, taken altogether, is greater now than it was in the times of Julius Caesar, the dispute would be at once determined.
When we are assured that China is the most fertile country in the world, that almost all the land is in tillage, and that a great part of it bears two crops every year, and further, that the people live very frugally, we may infer with certainty that the population must be immense, without busying ourselves in inquiries into the manners and habits of the lower classes and the encouragements to early marriages.
But these inquiries are of the utmost importance, and a minute history of the customs of the lower Chinese would be of the greatest use in ascertaining in what manner the checks to a further population operate; what are the vices, and what are the distresses that prevent an increase of numbers beyond the ability of the country to support.
Hume, in his essay on the populousness of ancient and modern nations, when he intermingles, as he says, an inquiry concerning causes with that concerning facts, does not seem to see with his usual penetration how very little some of the causes he alludes to could enable him to form any judgement of the actual population of ancient nations.

If any inference can be drawn from them, perhaps it should be directly the reverse of what Hume draws, though I certainly ought to speak with great diffidence in dissenting from a man who of all others on such subjects was the least likely to be deceived by first appearances.

If I find that at a certain period in ancient history, the encouragements to have a family were great, that early marriages were consequently very prevalent, and that few persons remained single, I should infer with certainty that population was rapidly increasing, but by no means that it was then actually very great, rather; indeed, the contrary, that it was then thin and that there was room and food for a much greater number.

On the other hand, if I find that at this period the difficulties attending a family were very great, that, consequently, few early marriages took place, and that a great number of both sexes remained single, I infer with certainty that population was at a stand, and, probably, because the actual population was very great in proportion to the fertility of the land and that there was scarcely room and food for more.

The number of footmen, housemaids, and other persons remaining unmarried in modern states, Hume allows to be rather an argument against their population.