In the 1970s, Gilles Deleuze and Félix Guattari produced three imposing books: Anti-Oedipus (1972), Kafka (1975) and A Thousand Plateaus (1980). With their raucous mockery of Hegelianism and Marxism, psychoanalysis and structuralism, these manifestoes for ‘delirium’ and ‘schizo-analysis’ were meant to be infuriating rather than ingratiating. They were like machines for wrong-footing their readers: anyone who queried them was obviously a chicken or a square. But in a perverse way they proved seductive too, not only in French but—perhaps especially—in the awkwardness of English translations: they were like a blast of fresh air, blowing away the cosy idea—very influential at that time—of a soft, interpretive world of human meaning as distinct from the hard-edged world of science.

But by 1991 the times had changed; and so, it appears, had Deleuze and Guattari. The mournful opening pages of Qu’est-ce que la philosophie? footnote—which was to be their last joint work—seem like a bid for sympathy, if not a cry for help. The authors present themselves not as terrible young tearaways, but as piteous relics of a bygone age, a frail Don Quixote preparing to utter his lucidly disillusioned last words. The time has come, they say, when there is nothing left for them except the question ‘What have I been doing all my life?’ Or in other words, what is philosophy? And this, they continue, is an issue which can be broached only by those who, like them, are heavy with age: a question for the lonely midnight hour of ‘quiet restlessness’, for the brief ‘moment of grace between life and death’.

Guattari died, aged sixty-two, shortly after the book was published; and Deleuze, who is five years older, has been famously unwell for years. You would need a heart of stone, then, to be unmoved by the valedictory miasma that rises from this final manifestation of their great double-act. In this last book, the improbable collaboration between a university philosopher (Deleuze is the author of studies of Hume, Kant, Bergson, Spinoza, and Nietzsche) and a rascally anti-psychiatrist (Guattari could never forgive Laing and Cooper for selling out to the capitalist therapeutic system), has changed its tune: this is not a call to arms by a post-Marxist revolutionary, but the melancholy memoir of a philosophical connoisseur.

What is Philosophy? is a testament of love: love for the great canon of Western philosophy—for Socrates and Berkeley, Descartes and Husserl, and above all for Spinoza, the thinker of immanence and saintly self-effacement, the ‘prince’ and the ‘Christ’ of philosophers. Deleuze and Guattari have no time for the kind of finger-wagging ‘historicism’ which keeps reminding us, with triumphant self-satisfaction, that the exalted canon of Western classics was assembled by sleazy processes of violence and exclusion. Of course it was: but they are not going to let that sour their delight. They like their philosophy the old-fashioned way—strong and fiery; and they pride themselves on taking it neat, unlike the lily-livered relativists and sentimental neo-Kantians who water it down for the sake of accessibility, interdisciplinarity, progress or edification. The dilute philosophizings of twentieth-century pedagogues, pragmatists and mediacrats may have brought ‘social benefits’, they say, but they have been ‘an absolute disaster for thinking’.

There is a second kind of historicism that they feel obliged to face down: the practice of interpreting philosophical texts as if they collectively formed a single tapestry of argument. Its main proponents were Hegel and Heidegger, philosophy’s great totalitarians, who seem to have hated the variousness of philosophy so much that they tried to force all the philosophers to sign the same confession, acknowledging complicity in a world-wide ‘metaphysical’ conspiracy. (‘Philosophy never establishes anything new,’ as Hegel once said; and Heidegger maintained that all true philosophers have ‘fundamentally said the same thing’.)