"There are things we know we know," said Donald Rumsfeld about Saddam Hussein's regime in Iraq. "We also know there are known unknowns... But there are also unknown unknowns." And with that now notorious comment, he paradoxically turned not knowing into a tongue-twisting tease of scientific certainty and precision, all in keeping with the black-and-white politics of the age. The choice is simple. Either you are a democratic, rational, capitalist country allied to America's perspective on the world that "knows" things or you are part of the axis of evil, driven by irrational fervour about which everything significant is already known. "You are either with us or against us in the fight against terror," said George W. Bush shortly after September 11 2001. The world must decide whether it supports America or the terrorists.

But Gayatri Spivak's book lays a claim to the "unknown unknowns" and strikes at the heart of Bush's and Rumsfeld's simple, political fundamentalism. Rather than promoting clearcut polarities between good and evil, which mask a frightening new imperialism, Spivak celebrates complexity, uncertainty and even, arguably, obfuscation.

"The 'planet' is a catachresis for inscribing collective responsibility as right," she concludes. "Its alterity, determining experience, is mysterious and discontinuous - an experience of the impossible." And, at the beginning, we are assured that she will do her "best to explain, but I am hampered by the fact that I am not out to demystify".

The immediate purpose of the book is to underline the importance of comparative literature as an academic discipline. Comparative literature departments are under threat across the US, partly, according to Spivak, because the supposed initial motivation behind their establishment no longer seems relevant. Formed by European intellectuals fleeing the horrors of totalitarian regimes in the 1930s, literature departments buzzed with cosmopolitan conversations, in which everyone shared the same experiences, if not the same language. Now, when half the class might be first-generation immigrants from third-world countries, such presumptions of universalism become increasingly problematic.

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The answer, Spivak argues, is to reinvigorate comparative literature by combining it with the best aspects of other academic departments, with area studies or cultural studies. Area studies, in isolation, is compromised as a discipline by its original establishment "to secure US power in the cold war". But, if combined with comparative literature, the impressive anthropological insights it can provide, with its history of careful fieldwork and of the rigorous observation of cultural differences, can be brought together with the linguistic competence and literary sophistication of the comparative literature department. Building bridges between these two departments would succeed in "displacing the discipline", which is always a desirable goal for Spivak.

But the book is about far more than university turf wars. It addresses the question of how we imagine the rest of the world. Central to this question is an appreciation of how others imagine us. And this is, crucially, something that we can never hope fully to understand or to know. Spivak draws on the example of J. M. Coetzee's Waiting for the Barbarians , where an imperialist magistrate hopes in vain to understand a "barbarian" girl.

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The meanings that the magistrate tries and fails to impose on the girl are instructive for our relationship with the rest of the world. "Our own undecidable meaning is in the irreducible figure that stands for the eyes of the other," Spivak writes.

Training in foreign languages can go some way, however, usefully to reduce these "irreducible figures". Spivak points out the political assumptions behind acts of translation, which can erase the "history of distinctions" in another country that might be the legacy of imperialist intervention.

Language bears the imprints of past losses and separations that must be attended to and the severity of atrocity and trauma may be weakened or disavowed in translation. It is important, therefore, to read the experiences of others in their own language. In this regard, Spivak calls for a widening of the number of languages studied in comparative literature departments to include southern hemisphere countries: languages from Africa and from Southeast Asia. Spivak's demands might seem idealistic and unfeasible to stretched university departments, but, in this climate of international intolerance and fundamentalism, they are nevertheless admirable.

More than anything, Spivak's book is a hymn to the imagination, to the vital political or ethical force it can wield in the world. Reading literature allows one to engage in the imaginative fields of unknown other people, which necessitates the crucial displacing of powerful hegemonies and distorting prejudice. And developing a literary sensibility means that one is content not fully to understand but to dwell in ambiguity and multiple possibilities.

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"We live in a time and place that has privatised the imagination and pitted it against the political," Spivak observes with alarm. Instead, the imagination should be brought back to its central public function as the driving and destabilising force within the state.

Death of a Discipline began life as a series of lectures - the Wellek Library lectures - delivered at the University of California, Irvine, and it shows signs, at times, of its oral, improvised origin. The writing is dense and the lengthy sentences, which were probably clear when performed orally with the correct emphasis, wind around their complex subject matter like prickly cats twitching their tails. The poor copy-editing does not aid the reader's understanding.

But perseverance is rewarded by all the riches in the book. My favourite is Spivak's alternative term for globalisation. The globe, she maintains, smacks of a cosmic imperialism. After all, it is only one of many globes in the solar system. Why not replace "globe" with "planet", which recognises the fact that it "belongs to another system and yet we inhabit it"?

Jennifer Wallace is a fellow of Peterhouse, Cambridge.

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