However this question should be answered the cases I have cited are sufficient to shew where the chief difficulty of the enquiry lies. Decadence, even if it be a reality, never acts in isolation. It is always complicated with, and often acts through, other more obvious causes. It is always therefore possible to argue that to these causes, and not to the more subtle and elusive influences collectively described as ’decadence,’ the decline and fall of great communities is really due. Yet there are historic tragedies which (as it seems to me) do most obstinately refuse to be thus simply explained. It is in vain that historians enumerate the public calamities which preceded, and no doubt contributed to, the final catastrophe. Civil dissensions, military disasters, pestilences, famines, tyrants, tax-gatherers, growing burdens, and waning wealth--the gloomy catalogue is unrolled before our eyes, yet somehow it does not in all cases wholly satisfy us: we feel that some of these diseases are of a kind which a vigorous body politic should easily be able to survive, that others are secondary symptoms of some obscurer malady, and that in neither case do they supply us with the full explanations of which we are in search. Consider for instance the long agony and final destruction of Roman Imperialism in the West, the most momentous catastrophe of which we have historic record. It has deeply stirred the imagination of mankind, it has been the theme of great historians, it has been much explained by political philosophers, yet who feels that either historians or philosophers have laid bare the inner workings of the drama? Rome fell, and great was the fall of it. But why it fell, by what secret mines its defences were breached, and what made its garrison so faint-hearted and ineffectual--this is not so clear. In order to measure adequately the difficulty of the problem let us abstract our minds from historical details and compare the position of the Empire about the middle of the second century, with its position in the middle of the third, or again at the end of the fourth, and ask of what forces history gives us an account, sufficient in these periods to effect so mighty a transformation. Or, still better, imagine an observer equipped with our current stock of political wisdom, transported to Rome in the reign of Antoninus Pius or Marcus Aurelius, and in ignorance of the event, writing letters to the newspapers on the future destinies of the Empire. What would his forecast be?