• Within one's own mind, a danger signal is the sheer emotional appeal of some belief systems when first encountered. The very fact that we desperately wish something to be true is a pointer to the possibility that we may select the evidence that seems to support our favoured belief and ignore whatever contradicts it. Many of us are guilty of attending only to arguments with which we already agree; we prefer to bolster our beliefs rather than to challenge them. And the more important the subject matter of the belief, the more we should be on our guard. If we have spent many years looking for the answer to a particular problem, we should be all the more cautious about accepting any apparent solution that may come our way.

• Totality belief systems have certain features that mark them out as what they are. One is the tendency to represent the universe as a giant cipher, to which they uniquely hold the key. We have to be careful here, because there is a sense in which mainstream science thinks of the universe in this way too. The difference between a scientist and someone suffering from the Casaubon delusion is not so much in the nature of their theories as in their readiness to submit their ideas to the test of experimental verification, but the distinction isn't always easy to make. One useful touchstone, I've found, is the attitude to questions.

• Within many belief systems you find an apparent readiness to accept questioning, and this may be quite impressive at first. However, this openness is usually confined within limits. Becoming a member of a group dedicated to the study and practice of such a system is rather like learning a new game, with very complicated rules many of which are never spelled out but have to be picked up as you go along. Peer group pressure is undoubtedly an important factor in such circumstances.

  Psychological experiments have shown that group attitudes can affect how people perceive things. For example, if you are shown two lines of equal length when you are a member of a group in which all the other participants have previously been told to say that the lines are unequal, it's quite likely that you, too, will perceive them as unequal.

• Because they believe they have discovered or been given the key to a mystery, adherents of a belief system tend to regard themselves as an elite. Of course, in-groups are found in all human organizations, but the very nature of organizations dedicated to the study of esoteric ideas means that inner subgroups with claims to special knowledge are particularly likely to arise.

• A feature of many belief systems is that they are said to be of great antiquity, even if they have apparently arisen quite recently. Just because the 'knowledge' professed by such groups is said to possess timeless verity, it can never change. It is therefore static. This doesn't mean it is boring for students; indeed, these students always feel their exploration of the esoteric knowledge to be immensely exciting. The leader of the group often ensures this by progressively revealing more and more of his ideas as time goes by.

• Another characteristic feature is that the knowledge is usually in the possession of an inspired teacher. Nearly all totality belief systems are equipped with at least one guru, who is normally the founder of the system. He or she may be dead, however, in which case the guru's mantle will have been draped on the shoulders of one or more disciples. In extreme cases, where the system is of vast antiquity, the guru will be a legendary figure, as in traditional Chinese acupuncture, where the founder is the mythical Yellow Emperor. The only way you can avoid encountering such a figure is by inventing your own system from scratch, in which case you will be the guru yourself if you enlist any followers.

• Perhaps the most characteristic feature of belief systems is the degree of conviction with which they are adhered to. We need to remember always that the amount of certainty that we feel about our beliefs is not a reliable guide to their correctness. The subjective sense of certainty is no guarantee of truth. Indeed, it's often when we find ourselves most firmly convinced of having attained ultimate truth that we have fallen most deeply into the toils of the Casaubon delusion.

See also The Casaubon Delusion and Living with Uncertainty