There are indeed some more particular physical causes of great importance. Diseases are mentioned in antiquity which are almost unknown to modern medicine, and new diseases have arisen and propagated themselves of which there are no traces in ancient history. And in this particular we may observe, upon comparison, that the disadvantage is very much on the side of the moderns. Not to mention some others of less importance, the smallpox commits such ravages as would almost alone account for the great superiority ascribed to ancient times. The tenth or the twelfth part of mankind destroyed every generation should make a vast difference, it may be thought, in the numbers of the people; and when joined to venereal distempers, a new plague diffused everywhere, this disease is perhaps equivalent, by its constant operation, to the three great scourges of mankind—war, pestilence, and famine. Were it certain, therefore, that ancient times were more populous than the present, and could no moral causes be assigned for so great a change, these physical causes alone, in the opinion of many, would be sufficient to give us satisfaction on that head.
But is it certain that antiquity was so much more populous as is pretended? The extravagancies of Vossius with regard to this subject are well known; but an author of much greater genius and discernment has ventured to affirm that, according to the best computations which these subjects will admit of, there are not now on the face of the earth the fiftieth part of mankind which existed in the time of Julius Cæsar. It may easily be observed that the comparisons in this case must be very imperfect, even though we confine ourselves to the scene of ancient history—Europe and the nations about the Mediterranean. We know not exactly the numbers of any European kingdom, or even city, at present; how can we pretend to calculate those of ancient cities and states where historians have left us such imperfect traces? For my part, the matter appears to me so uncertain that, as I intend to throw together some reflections on that head, I shall intermingle the inquiry concerning causes with that concerning facts, which ought never to be admitted where the facts can be ascertained with any tolerable assurance. We shall first consider whether it be probable, from what we know of the situation of society in both periods, that antiquity must have been more populous; secondly, whether in reality it was so. If I can make it appear that the conclusion is not so certain as is pretended in favour of antiquity, it is all I aspire to.