For aspiring-to-be-rational heathens like myself, texts such as Pope Benedict's Christmas address to the Roman Curia are often used as target practice for sharpening our critical thinking skills and BS radars. How easy it is to take a sentence like, "Only faith gives me the conviction: it is good that I exist," and reply, "Speak for yourself, mate." But if we look more charitably, the pope's speech provides an important insight into the limits of rationality.

The first key sentence is, "Only if God accepts me, and I become convinced of this, do I know definitively: it is good that I exist." On this, I think he is pretty much right. Of course secular humanists believe that it is good that human beings exist. But catch one on a bad day and she'll probably admit the world is a pretty screwed up place and it isn't obvious that it would have been better if our particular cosmic accident hadn't happened. Believe a good God created us, however, then although it's pushing things to say you "know definitively" (not much humility about human limitations in that assertion), your belief that it is good we are here is nearly as strong as your belief in the creator.

This leads to the second key sentence: "Where doubt over God becomes prevalent, then doubt over humanity follows inevitably." Again, I think he is right. Humanism is faced with the bind that its existence depends on maintaining a tension between finding what is good and worth celebrating in the human and having the intellectual integrity to see our species warts and all, which means being open to the possibility that we are not as great as we'd like to think we are. No self-respecting humanist can fail to have "doubt over humanity", and although that need not occlude all the light, it is a dark cloud we have to live under.

Here's where it gets interesting. The doubt over humanity that is an inevitable corollary of secular humanism cannot be neatly contained and eventually it spills over into doubt abut the capabilities of human reason. Indeed, the more you know about how the human mind works, the less reason we have to trust our rational capacities. For instance, Alvin Plantinga's evolutionary argument against naturalism claims that secular reason leads to evolution, but evolution removes any reason we might have to trust secular reason. There is no reason to believe that a brain that evolved to help us survive in the pleistocene is a reliable tracker of truth. Darwin himself had this concern, writing that "the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy".

This is a clever argument, but we have much less contentious reasons to doubt our rational capacities: decades of research in psychology have shown us to be unreliable, distorting, self-serving creatures who routinely reason with prejudice. The very smart are not immune. Indeed, they sometimes seem to be more capable of distortion than others, since they are clever enough to construct whatever argument they need to prop up what they already believe.

What all this suggests is that in practice there is no neat distinction between the logical and the psychological. Those who attempt to use pure reason cannot expect to succeed, while those who willingly allow psychological factors to affect their reasoning may be being more self-aware about their rational capacities than those who do not.

Return to the pope and his message becomes more interesting. Nothing in what he said appeared to provide any rational grounds for belief, only psychological incentives to believe. It is because it provides certainties about our ultimate worth that, as Benedict put it, "faith makes one happy from deep within". For atheists, the standard response would be: tough. We have to live being uncertain that it is good that we exist, with doubts about humanity. End of story.

But this might just be too quick. Kierkegaard saw the limits of reason as themselves a reason to make irrational leaps of faith. In a more modest form, his insight could help explain the rational non-rationality of much religious belief. Although not couched as arguments to believe, a lot of religious talk and writing is of the kind "without faith, we are lost". To say that this is not an argument for faith misses the point, because taken seriously, it means that without faith, trust in our capacity to argue rationally is lost too, and so we cannot rely on it alone to provide reasons to believe or not. If that is the human condition, then are we not justified in being pragmatic towards religious truth, and accepting faith because it seems to work? We choose faith so as not to be lost, because the alternative, reason, cannot enable us to find ourselves.

As an atheist, I'm not convinced by this. People who have a point are often nonetheless wrong, and often it's precisely because of that point that they go wrong. Reason has its limits but we need to go right up against them, and for my money faith sees these limits and gives up on reason too soon. Nonetheless, the mere fact that a serious argument can be made against the coherence of relying on human reason alone not only gives us atheists a way of understanding religion more sympathetically, it also suggests that the limits and role of reason has been a relatively neglected area of debate between believers and non-believers. Perhaps we have reason to believe that it might be a more fruitful one than some of the others that have dominated the conversation in recent years.