Freud once wrote, “Intolerance of groups is often, strangely enough, exhibited more strongly against small differences than against fundamental ones.” This is certainly true of intellectuals. The problem is that if you look at anything very closely, including ideas and ideals, differences which appear small from the wider perspective suddenly appear very large indeed. And so it should be. It is precisely our ability to examine the objects of intellectual endeavour closely and discern differences invisible to the naked mind’s eye which allows us to deepen and extend our learning in the humanities and the sciences.

However, if we never step back and examine the broader picture, we can become blinded to some important features of intellectual life which should be obvious to us. And while intellectual hyperopia gets in the way of first class, specialised academic work, intellectual myopia is a more pernicious and widespread affliction of intellectual life today.

Intellectuals in general, and academics in particular, have taken their eyes off the ball. They have forgotten how precious their shared commitment to rationality is. Instead, they fetishise technical disagreements and lose sight of one of the core intellectual virtues they share. The result is that the values of rationality and reasonableness become debased and we are left with no defences against the traditional enemies of enlightened humanism: superstition, ignorance, prejudice and plain stupidity.

Consider, for example, what Simon Blackburn, in the title of his forthcoming book, has called “The Truth Wars”. The general history of this conflict has been chronicled many times. First came the Enlightenment, and the championing of reason, truth and science over authority, falsehood and superstition. Then, in the twentieth century, many lost faith with the Enlightenment project. Some, such as Adorno and Horkheimer, went so far as to suggest that Auschwitz was the logical conclusion of the Enlightenment. We had to reject, so the story went, the myth of the rational mind, dispassionately analysing the world. To understand the history of ideas, we need to look not at syllogisms but at who wields power or at the subconscious mind. Reason does not determine what we think; rather, what we already think determines how we reason.

These disagreements are important within the academy today, which is divided between those who carry the torch of the Enlightenment and those who debunk its pretensions. But the disagreements occlude a more important agreement. As Bernard Williams put it in his swan song, Truth and Truthfulness, differences in each camp’s “respective styles of philosophy” leads them to “pass each other by”.

Williams identifies two virtues of truth that are central to intellectual enterprise: sincerity and accuracy. You do your best to understand things as clearly as possible and “what you say reveals what you believe”. Although Williams maintained that these virtues only make sense if there is such a thing as the truth, in some form or another they are held by virtually everyone with a commitment to intellectual work, with important consequences. For what it implies is a shared commitment to nothing less than rationality itself.

This fact is disguised by the failure to distinguish between thin and thick conceptions of rationality. To illustrate the differences, consider thin and thick conceptions of hedonism. The thin conception of hedonism is the minimum which anyone who is any kind of hedonist is committed to. This is roughly the idea that the best form of life is one in which pleasure or happiness is maximised. Such a thin conception leaves a great deal undetermined. In order to “thicken” the conception and come up with a well worked-out hedonistic blueprint for living, much needs to be added, and what you add can result in very different blueprints. Nonetheless, the thin conception does actually rule out a great deal. No kind of hedonist will make duty the cornerstone of their ethics, for example.

In just the same way, even though intellectuals and academics disagree about their thick conceptions of rationality, almost all agree on a thin one, in which the virtues of sincerity and accuracy are central. These two virtues manifest themselves in five features of rational discourse: comprehensibility, assessability, defeasibility, interest-neutrality and compulsion of reasons.

No thinker of any quality sets out to deliberately make their writings unnecessarily incomprehensible. If one is committed to the value of reason, then one is committed to explaining oneself in terms which are in principle comprehensible to others, even if such understanding is difficult. This is part of the sincerity of the intellectual enterprise.

It is also important that these comprehensible reasons are in principle assessable by others. So, for example, if I tell you that I have infallible knowledge and you just have to accept that; since you can’t possible know why that it is the case, I have given you a comprehensible belief, but not an assessable one. I have not, therefore, given you any rational grounds to believe I have infallible knowledge. My sincerity means that I am open to criticism and my commitment to accuracy opens up the possibility that I am wrong. Together, these entail the notion that what I say is assessable by others.

This is important, for the hallmark of rational enquiry is that it does not just provide the whats of belief, but also the whys. This is why so much religious teaching lies outside of rationality. If I am told that I must accept that something is true because it is the word of God, and that the reason it is the word of God is just that we have faith that it is, then I have not been provided with any rational grounds to believe that thing is true. There must be some way for me to assess the truth – or value – of what it is that is being claimed.

The third feature of rational reasons is their defeasibility. This is simply the fact that we always accept that it is possible we are wrong. It follows from the fact that our beliefs are assessable by others that their assessment may be unfavourable. We remain open to the possibility of our own error. The moment one says that such and such a principle can no longer be the subject of any doubt or sceptical inquiry, one has shut the door on rationality and turned belief into dogma.

Rational argument also strives to be as interest-neutral as possible. It is part of the sincerity of rational arguments that they are never knowingly glosses for partisan prejudices. This is the case even when the upshot of the enquiry is that no enquiry is ever truly interest-neutral.

The last, and most elusive, of the features of rationality I see as central is compulsion. This is simply the feature of rational reasons for belief that, if they are strong and understood, they exert some kind of force upon you. You don’t just decide to agree or disagree with good reasons for belief. You feel in some sense compelled to agree or disagree with them. This relates to the virtue of accuracy: a good rational argument is answerable to something other than our will.

Why does any of this matter? Consider how someone might object that this thin conception of rationality is too thin. Even religion, for example, can offer reasons for belief that are comprehensible (albeit not necessarily in traditional, rationalist terms), assessable (albeit it not by entirely rational means), defeasible (Kierkegaard talked of the risk of faith, the risk being precisely that we could be wrong), interest-neutral (even though those who grow up in a different religious tradition may find the transition hard), and which certainly exert a compelling force on those who come to accept them.

There is no problem here. Since it is evident that there are many religious people who have not given up on rationality, any thin conception of rationality must allow the religious in. The point is that, once in the big tent of rationality, then more detailed debate can begin on how to thicken the idea of rationality, and it is at this stage, and only this stage, that sensible disagreement about the rationality or irrationality of religious belief can begin.

This is why a thin conception of rationality is so important. The true spirit of the enlightenment is not to be found in any specific beliefs about the power and scope of rationality. Rather, the key is that disagreements can be discussed and argued, in a common, intellectual space in which everything is open to everyone and no appeals to authority are allowed to trump. This broad domain of rationality is very precious indeed.

The problem is that, to listen to many western intellectuals, you would think that it doesn’t exist. This, even though many earn their living at universities that are concrete manifestations of institutions committed to thin-rationality.

If we are tempted to think that thin rationality is too woolly a notion, we should remember that there are many people who do reject it. To use another word with unfortunate connotations, fundamentalists of all descriptions have opted-out of the domain of rational enquiry. What they hold to be true is certain, not defeasible. It is assessable only by God, not man. There is no attempt to understand the interests of others who take a different view, and the compulsion they advocate comes not from the ideas themselves, but the force of violence.

More worryingly, I find it impossible to ignore the parallels between what I have been describing and the attitude of the United States in particular towards Iraq. Whether or not it was better to confront Saddam Hussein’s regime, the manner in which the US government conducted itself seems to me to mark a disturbing disdain for the values of rationality I have defended. I am not convinced that the administration had any interest in making its true reasons for action comprehensible. Its main claims about Saddam Hussein were not genuinely assessable but had to be taken on trust. There was a certainty of purpose which seemed to me to ignore the demand that we accept the defeasibility of our beliefs. Nor was enough attention paid to the interests and perspectives of others who took contrary views. And finally, it did not matter that the reasons given were not compelling; compulsion was achieved militarily. In short, a commitment to sincerity and accuracy, and with it rationality, seemed to be sorely lacking in the US administration.

And yet the academics and intellectuals who were among the strongest critics of the war need to accept some responsibility for eroding the rational domain and helping to make this disregard for it possible. Our obsession with our scholastic disagreements has created the impression that there is no common domain of rationality within which disagreements can be thrashed out. We just have a multiplicity of discourses and rationalisations to legitimise different interest groups.

I should make it clear that this is not just a criticism of those currents of thought broadly and loosely labelled post-modern. The enemies of post-modernism have set themselves up as the sole champions of reason – something made easier by their opponents’ willingness to relinquish the labels of rationality and reason. In so doing they too have contributed to the sense in which the intellectual sphere is too fragmented and divided along factional lines for any general dialogue to be possible. By dismissing large sections of the intellectual community as anti-rational, the anti-postmodernists have also contributed to the sense in which it is pointless to seek to argue one’s case in the widest possible forum.

We need, therefore, to reassert our shared values of rationality, to relegitimise the domain of intellectual discourse as the right place to discuss differences and settle disagreements. It is time western intellectuals took a wider view and realised that unless they stress what they have in common, the whole enterprise of rational enquiry will only come to see more and more irrelevant to those who seek solutions to the problems of today.

Based on a talk given to the seminar ‘Humanism for the 21st century’, Perspectives East and West Conference, at the Centre for Development and the Environment, University of Oslo, 4-5 June 2004.

Julian Baggini is author of What’s It All About? Philosophy and the meaning of life.