The man who is widely regarded as having made a career out of pissing on English literature has gone and done that very modish thing of writing a memoir. It is an odd thought that Terry Eagleton - whose ardent practice of literary theory in 1970s Oxford turned him into a kind of devil doll for a generation of Leavisites - was once a little boy. But here he is, on the front of his book, tiny and sweet and wearing what look like Clarks sandals - but probably aren't, because his parents would never have been able to afford them.

Eagleton was born in wartime Salford, the grandchild of immigrant Irish labourers. His description of the surrounding slumminess has a boastful Monty Python quality to it (starving schoolmates puking up their lunch, little Terry taunted as a toff because he wears a winter coat). This was a world where, if Eagleton is to be believed, initiatives such as the National Health Service and the 1944 Education Act made no dent in the enduring narrative of despair that was the lot of the urban working classes. Except that, in Eagleton's case, there did turn out to be a different ending. Unlike his two elder brothers, the chronically wheezy Terry survived into adulthood, and was able to take up his place at the grammar school. There was, after all, a way out of the never-ending story.

If Eagleton's home life was a sludge of dun tints, his parallel existence, as an altar boy at the local Carmelite convent, was one of screaming colour. There was a nun with ginger whiskers, a cross-dressing lay server, and any amount of elaborate ritual, doggerel Latin and throat-catching incense. As a pre-pubescent boy, Eagleton acted as "gatekeeper" between the private and public parts of the convent, which were connected by a series of Hammer Horror sliding doors, secret drawers and crazy passages. Very small and gowned, it was his job to usher the parents of noviciates into the closed convent to say a final farewell to their teenage girls.

Eagleton makes this early experience of transplanted Irish Catholicism fundamental to his subsequent intellectual development as a Marxist. The utter lack of interest in interiority, compromise or shades of difference that he absorbed at the convent prepared him perfectly for the dogma that he was to encounter at all those leftist political meetings in the Sixties and Seventies. Marxism, like Catholicism, requires one to be either inside or outside the fold. Liberalism, with its endless anguish, reflexivity and shifting boundaries, has no need for gatekeepers.

Cambridge in the early Sixties, when Eagleton arrived to read English, turned out to be another absolutist kingdom. For all its liberal provenance, the place was as tightly walled as a Carmelite convent. Undergraduates were over-whelmingly tall, assured and convinced of their right to be there - none of which applied to Eagleton. Dons were self-appointed custodians of a cherished body of culture, which included everything worth knowing from Rubens's brush technique to 17th-century military strategy by way of the manufacture of saddles. Now it was Eagleton's turn to present himself to a gatekeeper - in this case, Dr Greenway, his supervisor. Greenway, who claimed to have been offered the Trinity College English fellowship while practising as a lawyer, exactly captured the gentlemanly, generalist approach to the study of literature that Eagleton was to devote his stormy career to clawing down.

Unfortunately, Eagleton doesn't tell us much about the ensuing battles between himself and the various creaky institutions that he has annoyed along the way - the one incident he describes in detail has more to do with the menu at High Table than with the ins and outs of post-structuralism. Instead, he takes a defiantly patchwork approach to the story of his life, moving from the personal to the general, the anecdotal to the theoretical, sometimes within the same paragraph. There are sections on nuns' knickers, Wittgenstein, a first date, picketing, two funerals, Maurice Bowra, fogeyish undergraduates, as well as his grandma who looked like Seamus Heaney in drag.

Is this because Eagleton is writing what he refers to, in a teasingly buried passage, as "anti-autobiography"? This, he explains, would involve writing the story of your life in such a way "as to outwit the prurience and immodesty of the genre by frustrating your own desire for self-display and the reader's desire to enter your inner life". Certainly, The Gatekeeper tells us little about Eagleton's demons and passions, and even less about his siblings, children and wives - all considered essential staging posts in traditional autobiography. More questionable is whether Eagleton manages the bit about frustrating the desire for self-display. He can't quite resist telling us about the exhibition and scholarship that came his way after gaining a place, against the odds, at Trinity, as well as about the fake snog that he and a fellow activist employed to distract the police from searching their car for seditious literature. He is clever and funny and sexy, and no amount of self-deprecation will hide the truth that he knows it, and wants us to know it, too.

As a result of this desire to stay on the surface (another legacy of both Catholicism's and Marxism's suspicion of subjectivity), the book often reads like a series of smoothly funny anecdotes that appear to invite intimacy while at the same time resisting it. On the strength of The Gatekeeper, Eagleton would make a fabulous guest on Parkinson, the kind who gets a whole show to himself. Only his simmering loathing of postmodernism, with its fatal lack of interest in the long, slow cycles of history, would stop him doing it.

Kathryn Hughes is a biographer and critic