Geographer Edward Soja has found postmodernism a powerful critique that has advanced his subject. He tells Huw Richards about the 'third possibilities' that traditional theories missed. Edward Soja has no problem with being described as a postmodernist. His book Postmodern Geographies: The Reassertion of Space in Critical Social Theory (1989) has been among the most influential works in advancing postmodern analysis in one of the subjects where it has had greatest impact.

He is, though, considerably less happy with many things written in the name of postmodernism: "I am not attracted by the excesses of people who extend the postmodern critique into anti-modernism, anti-Marxist and anti-science views, take up positions antagonistic to science and social science or dwell on the playfulness of the postmodernism critique in order to pastiche or hype."

Such effusions, he notes, are seized on by critics of postmodernism as ammunition for their polemics. But Soja, professor of urban planning at the University of California, Los Angeles (although he spends one term a year at the London School of Economics), is quick to dismiss allegations that postmodernism has no practical uses while noting that some traditional social scientists have tended to get bogged down in theories to the detriment of their practical applications.

He says his purpose has always been practical: "I aim at a critique of power and at seeking ways of changing the world for the better." He places great emphasis on the word "critical" in explaining his relationship with the traditional social sciences. He does not reject them - though he is scathing about the propensity of economic modellers to over-theorise ("if the model does not fit with reality, reality is deemed to be wrong") - but points instead to what they miss.

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Soja argues that partly because of a misplaced desire to emulate the physical and biological sciences, social science pursued theories that would explain the social world as comprehensively as biology and physics can explain theirs. He says: "This does not mean that these master narratives are useless or wrong. But by attempting to explain everything, much that is significant or worth exploring becomes submerged."

Social science has been, he says, prone to see the world in binary either/or terms. As he told one interviewer: "I often answer either/or questions with both/and also" - refining the traditional thesis-antithesis-synthesis structure by putting previously unconsidered third possibilities into the equation."

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And while those unconsidered possibilities include those most emphasised by traditional, Marx-influenced, leftish academics - race, class and gender - Soja's most distinctive contribution to the social sciences has been his emphasis on spatial factors: "Since the mid-19th century, historical factors have been privileged over spatial ones. My argument is not just that space matters, but that it is a powerful force for shaping society - that the spatial shaping of society is as significant as the social shaping of space."

It was this view that led him to postmodernism. "I realised that it was a powerful critique not only of traditional social science and scientific geography, but of Marxism as well" - and one that offered a more satisfying and complete description of the world.

Soja regards himself as more a regional than an urban geographer, arguing that one has to look beyond individual cities. He says his base in Los Angeles has helped him to see the city as presenting "the first signs of things that will shape everywhere. It is somewhere you can see the general forces - from globalisation down - that also affect London, Singapore, Sio Paolo and even Scunthorpe, pretty much as they begin."

A particular interest is the development of regional pressure groups and coalitions such as the Bus Riders Union. This group, representing the mostly immigrant, low-income Angelenos who rely on public buses, won a case against plans to build a system that favoured wealthier users and districts by arguing that their geographical rights were being violated.

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The decision, Soja notes, could have repercussions for regional transport planners everywhere, including London. He expounds his view on London in TV's Panorama next month, when he will argue that a way must be found of tapping into the wealth of the City, to address the poverty that surrounds it and to provide housing and transport for the low-paid workers who are as important to the City's continuing prosperity as the high earners.

This doubtless sounds the sort of issue that traditionally preoccupies social scientists. And that, Soja would certainly argue, is the point - not to displace social science, but to provide better answers to its eternal questioning of how the world works.