There can be few people who are not occasionally prompted to puzzled reflection about human beings and their actions. Perhaps a long-standing friend does something that strikes us as wildly out of character, even quite incomprehensible. It is brought home to us how resistant to our understanding other people can be; we are forcefully reminded that another person's mental life is largely hidden from us. And we may well begin to wonder whether we can ever really know what someone else thinks and feels. After all, when we ascribe thoughts and feelings to another person, it seems that all we have by way of supporting evidence is what we observe, namely his external behaviour. But how can we know in the case of someone else which outward behaviour patterns are signs of which inner states? Perhaps the similarity of his behaviour to our own may lead us to conjecture that he thinks and feels as we do; but are we ever really entitled to be sure what is going on in his mind?

Such sceptical thoughts come to us very readily; so too do speculations about the nature of whatever it is that has those elusive inner states. And here we seem to be torn in two directions. On the one hand, thinking and feeling surely require the possession of a self or soul or mind, which we tend to think of as something distinct from a gross material body.