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. More recent writers, seeing that Montesquieu's aphorism was a vague proposition which meant nothing, have gone a step further, and approached much nearer to the real explanation of the phenomenon. Guizot, Sismondi, and Michelet have concurred in assigning as the real cause of the decay of the Roman empire, the prevalence of slavery among its working population, and the great and increasing weight of taxes to support the imperial government. There can be no doubt that these were most powerful causes of weakness; and that they stand prominently forth from the facts recorded by contemporary annalists, as the immediate and _visible_ causes of the decline of the empire. The history of these melancholy periods is full of eternal complaints, that men could not be got to fill the legions, nor taxes to replenish the treasury; that the army had to be recruited from the semi-barbarous tribes on the frontier; and that vast tracts of fertile land in the heart of the empire relapsed into a state of nature, or were devoted only to pasturage, from the impossibility of finding cultivators who either would till the land, or could afford to pay the taxes with which it was charged. Doubtless the large proportion--at least a half, perhaps nearly two-thirds--of the people who were slaves, must have weakened the elements of strength in the empire; and the enormous weight of the direct taxes, so grievously felt and loudly complained of,[5] must have paralysed, to a very great degree, both the industry of the people and the resources of government. But a very little consideration must be sufficient to show, that these were not the real sources of the decline of the empire; or rather, that if they had not been aided in their operation by other causes, which