‘Revolution’ in its fullest sense should mean a vital transformation of society, something far beyond any ordinary shaking-up, a decisive transfer of power both political and economic. This can happen very seldom. But Europe has known many lesser eruptions, resembling it in some measure. Overt class conflict has been only occasional, disharmony among classes or social groups has been ubiquitous, and in all these collisions its presence can be traced. They have been of many species; and with Europe's always uneven development growing more and more uneven, forms of revolt belonging to distinct stages of history might be going on in different regions at the same time.

Europe's mutability must be traced to fundamental features of its makeup. Underlying them has been its duality, the discordant nature it derived from its Roman-Christian and Germanic-feudal ancestry. Both strands made for close interweaving of state and society, in most of Asia joined by mechanical, external clamps. Both gave rise to a wealth of political or politically relevant institutions of all kinds, which forces of change could work on and through, even if often obstructed by them. In Europe also, unlike most of Asia, many autonomous polities packed close together meant that exterior factors were always liable to intensify internal frictions.

In medieval times movements of revolt included those of peasants against landowners, towns against lords, urban workers against employers in centres of nascent capitalism, peoples against foreign domination. Most strident of all was the example set by the dominant class itself, the feudal lords.