What, then, were the causes of decay which proved fatal at length to this immense and enduring dominion? Philosophers in all ages have pondered on the causes; but those hitherto assigned do not seem adequate to explain the phenomenon. Not that the causes of weakness are baseless or imaginary; on the contrary, many of them were most real and substantial sources of evil. But what renders them inadequate to explain the fall of Rome is, that they had _all existed, and were in full operation, at the time when the commonwealth and empire were at their highest point of elevation_, and centuries before either exhibited any symptoms of lasting decay. For example, the ancient historians, from Sallust downwards, are loud in their denunciation of the corruption of public morals, and the selfish vices of the patrician classes of society, as being the chief source of the decay which was going forward, while the growth of the republic had been mainly owing to the extraordinary virtue and energy of a small number of individuals.[4] But the very circumstance of these complaints having been made by Sallust in the time of Augustus, and the fact of the empire of the West having existed for four hundred, that of the East for fourteen hundred years afterwards, affords decisive evidence that this cause cannot be considered as having been mainly instrumental in producing their fall. How is the unexampled grandeur and prosperity of the empire under Nero, Adrian, Trajan, and the two Antonines, whose united reigns extended over eighty years, to be explained, if the seeds of ruin two centuries before had been sown in the vices and corruption of the rich patricians? In truth, so far was general luxury or corruption from being the cause of the ruin of the empire, the cause of its fall was just the reverse. It was the excessive _poverty_ of its central provinces, and their inability to pay the taxes, which was the immediate cause of the catastrophe. The nobles and patricians often were luxurious, but they were not a thousandth part of the nation. The people was miserably poor, and got more indigent daily, in the later stages of its decay.
Modern writers, to whom the philosophy of history for the first time in the annals of mankind has become known, and who were aware of the important influence of general causes on social prosperity, independent of the agency of individual men, have assigned a different set of causes more nearly approaching the truth. Montesquieu says, the decay of the Roman empire was the natural consequence of its extension. This sounds well, and looks like an aphorism: but if the matter be considered with attention, it will be found that it is _vox et præterea nihil_. Those who, with so much complacency, rest in the belief that the fall of the Roman empire was the natural result of its extension, forget that its _greatest prosperity was coexistent with that very extension_. It is impossible to hold that the decay of the empire was the consequence of its magnitude, when the glorious era of the Antonines, during which it numbered a hundred and twenty millions of inhabitants under its rule, and embraced nearly the whole known habitable globe within its dominion, immediately succeeded its greatest extension by the victories, unhappily to us so little known, of Trajan.