It's difficult now to believe that such a trivial incident prompted my first doubts about the existence of God. Religious instruction at the Christian Brothers school I attended in Liverpool was after all a daily component of the timetable and on that morning back in the fifties there was nothing at all unusual about the manner in which Brother Francis was instructing his fourth form on the incontrovertible truths of Catholicism. His theme on that particular day was the resurrection and we'd reached the familiar point in the story where the "risen" Jesus had paid a surprise embodied visit to his disciple, Thomas.

At that point, Brother Francis removed his glasses and gave them a vigorous polish, a sure sign that we were approaching the theological nub of the lesson. "What did Thomas do when he was confronted by the living Christ?" he asked. Someone in the class dutifully explained that Thomas put his hands into Christ's wounds. "And why did he do that? Yes, Humphreys?". The working class boy in the desk behind me who spent most of religious instruction practising his handwriting decided he was not going to waste too much calligraphic time on such an obvious question. "He was sceptical." That sounded fair enough for me. But it was immediately clear that this was the wrong answer. "He was not sceptical", boomed Francis. "He had doubts. I don't want any of you using that word 'sceptical'. Is that quite clear?"

That was the breakthrough. Even though until that stage we'd all happily bought into the esoteric beliefs of Catholicism, we somehow bristled at the idea that we could not use a particular word. From then on Humphreys and myself and two other classmates, christened ourselves "The Sceptics", and rather like beachcombers let loose for the first time with a metal detector, began to rake through other parts of Catholic doctrine in search of items that might register on our scepticism scale. Humphreys pronounced himself sceptical about the virgin birth. My speciality was the one about Jesus being the son of God. We didn't merely announce such scepticism. We consciously set out to discover supporting arguments. Humphreys passed round his copy of Joyce's Portrait of the Artist as a Young Man with its courageous account of the psychological consequences of disbelief.

I gleefully reported a conversation I'd heard on The Brains Trust about the incompatibility of Catholic creationism and the Darwinian theory of evolution. We needed as much material as we could get. The Catholics were after us. My mother demanded to know how there could be no God when nature was so beautiful. My girl friend, Veronica, wanted to know how it could all have started without the intervention of a Supreme Being. (I recorded the debate and its result in my diary. "Told Veronica that all her concern about beginnings was a product of language. It was language that created endings and beginnings not nature. Result. Laurie 9, Veronica 3"). I'm happy to admit that my scoring system was suspect but I have no doubt at all about the argumentative excitement of those times, the exhilaration of point and counterpoint. Like other rationalists and humanists I object profoundly to the idea of compulsory religious instruction at school, but I am still stuck with the paradox that I'm much more comfortable nowadays with those who have undergone similar battles against indoctrination than with those who simply dismiss religion with a wave of the hand.

But this isn't solely an argument about religion. It's an argument about the peculiar pleasures of confronting any form of holistic ideology, any total system of thought, any meta-narrative. Consider behaviourism. I first encountered this ideology when I enrolled as a mature student of psychology at Birkbeck College. I'd chosen psychology carefully. I was anxious to read a subject that promised some solution to the problem of why so many people still clung to metaphysical beliefs. More specifically, what was it about the human psyche that inclined it to attribute all the finest potential qualities of humanity to some transcendental being? Behaviourism certainly promised an answer. I was surrounded by lecturers who were convinced that this new approach to understanding human nature, an approach based on the apocalyptic writings of B. F. Skinner, would finally sweep away all the mentalistic introspective clutter that had previously made psychology appear little more that a poor relation to philosophy. The human mind, argued the behaviourists, first encountered the world as a tabula rasa. There were no innate mental predispositions. All errors and misunderstandings were the result of inappropriate conditioning. Emotional life was bound by the same rules. Emotions were simply physiological disturbances that could become attached over time to a wide range of different stimuli.

Once again I happily immersed myself in this new ideology. All went well until a second year seminar on the physiological correlates of emotion. Our tutor had described the manner in which the secretion of adrenaline and noradrenaline invariably accompanied feelings of fear and anger. In time, we were informed, similar bodily correlates would be found for all emotional states. I'd been aware throughout this exposition that my friend, Denis Gahagan, was vigorously putting question marks alongside each line of his notes. It was, though, only when our tutor allowed himself a little self-satisfied smile at the prospect of the future triumph of behaviourism that Denis felt sufficiently moved to speak. "But what about jealousy and envy?" he asked. "Will we find their physiological correlates?" "Eventually," said our tutor. "But", persisted Denis, "How will we know that we've done so without an analytic appreciation of the difference between jealousy and envy? We can't simply take someone's word that they are feeling jealous or envious without also establishing that they know the difference between the two states. And if the difference is an intellectual distinction how could such a distinction be represented at the physiological level?"

My friend's intervention may have been more intellectually challenging than Humphreys's characterisation of Thomas's attitude to the dead Christ, but it was, once again, an intervention that prompted doubt about an entire system of thought. From then on, Denis and I diligently hunted for evidence that human beings were something more than the sum of their conditioned reflexes. (Nothing made quite such an impact upon us as a short paper by Noam Chomsky which ridiculed the behaviourist explanation of language development). Once again there were arguments late into the night with Skinnerians who clung to the behavioural promise of human perfection through environmental manipulation with all the tenacity that had once characterised Catholic assertions about humanity's essential imperfection being guaranteed by the legacy of original sin.

After my four years at Birkbeck, I was appointed as a junior lecturer at the University of York. I rather expected my first seminar to be a bit of a doddle. My simple aim was to introduce the group to Erving Goffman's account of social interaction. But I'd naively failed to notice that at least three members of the seminar were mature students. They let me rattle on for about half-an-hour without interruption but then one of them could contain himself no longer. The seminar, he told me, had no connection to sociology. Goffman's thesis about the manner in which people presented themselves in public failed to recognise the socio-cultural circumstances that obliged people to behave in such other-directed ways. It took me another three seminars to realise that I'd accidentally fallen into a nest of revolutionary socialists. They were incredibly well read. They could quote Marx and Trotsky and Lenin with quiet authority and were utterly serious about their beliefs, totally committed to the coming revolution of the proletariat. The tables were slowly turned. They began to give me reading lists and issue invitations to their own meetings.

Within two years I'd become a member of the International Socialists and was taking a crash course in Marx. I soon realised that I'd joined something more than just another talking shop. Our job, I learned, was not like pre-Marxist philosophers, merely to analyse the world. Our immediate task was to change it. And the only way in which that change could be effected was by raising the consciousness of the working class, backing its struggles, nudging it ever closer to the barricades. It was a matter of some regret, therefore, that we only had one genuine "worker" in our group. Dunlop Griffiths worked at a nearby power station and was our proletarian touchstone. From time to time I detected a faintly ironic note in his contributions to the discussion but decided that this was nothing more than the understandable wariness of a "worker" in the midst of several dozen assorted intellectuals. But one evening when we were discussing the poor response of local council tenants to our call for a rent strike, Dunlop suddenly found his voice. "Why should they want to go on strike?" he asked. "They don't want that sort of trouble. They want a quiet life." This was heresy. How could the proletariat, the essential agent for the revolutionary overthrow of capitalism, ever be allowed a quiet life? They had a job to do. Dunlop happily rode the mounting intellectual indignation. "I can accept nearly everything about Marx's economic analysis. But what I can't buy is his prophetic certainty, his belief in the revolutionary qualities of the working class. They may have been bought off by capitalism but they show every sign of staying bought." The following week I skipped the meeting and went for a long drink with Dunlop.

I suppose I should regard each of these episodes as moments of liberation, moments when certainty was, after much ferocious arguments and animated debate, replaced by a fluid easy-going agnosticism. But that's only half the story. For while I may not miss my former ideological commitments, I do miss the sense of being among people who regarded life as a constant search for a theory or a system that would explain human nature and human behaviour. Life now seems extraordinarily free from that sort of commitment and the arguments it necessarily engendered. There are, of course, many people who hold strong views about such matters as environmentalism and globalisation. But a challenge to these views rarely initiates an argument. Far too frequently their proponents take shelter behind a sort of eclectic individualism. You're welcome to your own views on the matter you'll be told, but please don't challenge mine. They're not up for debate, they're part of my identity. In such conditions there's no room for a Humphreys or a Gahagan or a Griffiths. People who produce arguments that threaten to undermine whole systems of belief are quite irrelevant when those systems are not embraced for their logical coherence but because of their emotional affinity with one's sense of self: "I feel, therefore I know."

I recognise that the meta-narratives of Catholicism and behaviourism and revolutionary socialism have had tragic unintended consequences. But they also provided endless opportunities for productive debate about the idea of a good society. They were punching bags that helped to keep one in sound intellectual shape. How different from the fluffy all-embracing ever-yielding vacuities of third way politics and the so-called "new religions". Perhaps this explains why I sometimes feel uncomfortable with the laid back nature of contemporary humanism and rationalism, why I felt a surge of old-fashioned passion whenever the late Nicolas Walter entered a media debate, or when I read Anthony Grayling's recent fiery attack upon New Labour's plan to subsidise more religious schools.

Years ago, when I asked a colleague at York for a definition of a "humanist", he told me that "humanists were people who met in scout halls and sang hymns about not believing in God." Am I the only one in these bland times, who'd welcome some humanist hymn sheets, some programmatic declaration of beliefs and aims that might not have the apocalyptic quality of the meta-narratives I've described but at least suggested that here was an opponent looking for a good heavyweight scrap?