Nairobi has a growing reputation as one of Africa's most dangerous cities. However, when I recently passed through it (as briskly as possibly) it wasn't fear of attack that bothered me but the incessant approaches of generally polite but persistent touts and hawkers. At one stage, half a dozen safari salesmen waited for me outside a café for half an hour in order to resume their pitch when I had finished my break.

At such times the desire to be left alone is more powerful than the urge to reason well, and it becomes easy to think of these people as fraudulent and dishonest pests. But the truth is rarely so simple. If we want to understand why people behave as they do we have to be able to override the temptation to pass easy judgement. The so-called 'principle of charity' helps us to do just that.

The principle is unfortunately named, since we think of charitable acts as voluntary and optional, not as obligations or duties. But the principle of charity is all about giving people or ideas their due. It is not about being any more accommodating or generous than is fair.

As applied to the behaviour of others, the principle tells us to try and understand it in terms which make it as rationally comprehensible as possible. It isn't difficult to do this in the case of the hawkers and to see how being poor and surrounded by comparatively rich tourists might make a person inclined to try a little harder than usual to gain their business. No one would choose to persist for hours with a clearly unwilling potential customer, in competition with several others, unless they really needed the commission they had only a slim chance of getting. Their irritating behaviour is not best understood by attributing any exceptional failing or goodness on their part, but by attempting to understand how someone like us might have done the same thing.

All this might sound a little fluffy around the edges. It isn't, for two reasons. First, it is not a simple injunction to see people's actions in the best possible light, but to see them in the most comprehensible light. Sometimes, the most rational interpretation of someone's behaviour is that they are nasty, sadistic or cruel. The second reason is more significant, for to follow the principle properly requires us not to dismiss, say, the religious, as just misguided fools. We stand no chance of genuinely understanding the religious if we simply assume they have missed something obvious or made a logical mistake or two. Unless we can comprehend why people not too unlike ourselves might be religious we stand no chance of understanding the religious themselves.

In philosophy, the principle of charity has a slightly different but related meaning. The idea here is that we interpret, not people's behaviour, but intellectual positions or theses in as rational a light as possible. The aim is to avoid the straw man fallacy - rejecting positions not on the basis of their true characteristics but on the basis of crude or otherwise erroneous caricatures of them. (See below)

The two forms of the principle of charity - as applied to intellectual positions and to behaviour - share an important insight in common: other people are not usually as stupid or as different to us as we often like to think. Sometimes they are just unpleasant or they do have very different fundamental beliefs about the world. But we should not start with the assumption that either of these are true. We should only accept the existence of irreconcilable gulfs in thinking or human nature after we have tried as hard as we can to understand differences in a way which makes sense of our shared human nature.

Received wisdom would have it that the 'third way' is some intellectually vacuous halfway house between old left and right which lacks any substance or coherent rationale. Applying the principle of charity would require us to question this easy dismissal. Could it be that this particular third way is little more than a straw man, a caricature which bears little relation to the real thing? Shouldn't we assume that if a noted sociologist like Anthony Giddens is behind the third way, then it is likely to be more than just a sound-bite direct from Millbank? Have we even tried to find out what the third way really is first-hand, and not through the filter of sneering columnists? If we did we might find it rooted in an analysis of the social context of what Giddens calls 'late modernity', which in itself might give us pause to take the idea more seriously. The principle of charity does allow us to ultimately dismiss the third way, but only after having genuinely attempted to understand it in as rational a light as possible.