There is something comic in the unflagging appeal by political thinkers to the belief that history is a process of dialectical development. In its original meaning in Greek philosophy, dialectic meant the give and take of argument. There was no suggestion that it applied outside the sphere of philosophical reasoning, and Socrates - its supreme practitioner - did not imagine that the logical structures of argument would ever be replicated as large-scale movements in history.
The modern idea of dialectic came later, when Hegel fused Greek logic with the Christian belief in history as a redemptive narrative. History came to be seen as an inherently rational process, each phase of which includes and transcends all that has gone before. The charm of historical dialectic is that it underwrites progressive hopes. Hegel's fantasy captivated Marx, and generations of western leftists looked to the Soviet Union as embodying the next stage of historical development: secular, socialist and internationalist. After an interlude of more than 70 years, Russia has returned to Orthodoxy, Eurasian geo politics and a type of capitalism not too different from that of the late-tsarist period. Today, many of the same leftists look to the US as the vanguard of the global civilisation they once foresaw in the Soviet Union - a deliciously barmy view at a time when the US is in the grip of Christian fundamentalism and US power is in steep decline.
As the Greeks understood, history is a process of drift; where there are patterns to it, they are commonly cyclical rather than dialectical. Yet there are times when something like a dialectical transformation can be glimpsed amid the chaos. Such a shift seems to be under way in the microcosm of the British two-party system.
Margaret Thatcher aimed to destroy socialism in Britain, and in this she succeeded; but she thereby removed the enemy in relation to which her party had defined itself for most of the 20th century. With their reason for existence gone, the Conservatives began to fall apart. New Labour completed the process: by defining itself in Thatcherite terms, the party removed the last remnants of the Conservatives' identity. At the same time, Labour set a limit on its political viability and guaranteed its own demise.
While new Labour remains locked in the rhetoric and policies of the 1980s, the Tories have moved on. Aiming to recreate the stodgy, deferential Britain of her childhood, Thatcher created the highly fragmented and individualistic country we have today. By making peace with the society she unwittingly brought into being, David Cameron has signalled the end of the Thatcher era, and thereby of new Labour, an unalterably Thatcherite construction. He has no intention of reversing any of Thatcher's - or Blair's - core policies; indeed, he is likely to extend them. Yet by consigning Thatcher to the memory hole, he has made new Labour redundant and ensured its collapse. In a process that will surely delight all who believe in the logic of history, the Tories have emerged as the dialectical negation of That cherism that is poised to take Thatcherism to an even higher stage of development.