This book is organized around the “belief-desire model” of action. To understand how people act and interact, we first have to understand how their minds work. This is largely a matter of introspection and folk psychology, refined and corrected by the more systematic studies carried out by psychologists and, increasingly, by behavioral economists. The model is vital not only for explaining behavior, but also for assigning praise, blame, or punishment. Guilt usually presupposes mens rea, intentions and beliefs. Strict liability – guilt assigned merely on the basis of the actual consequences of action – is rare. In fact, sometimes we hold people guilty merely on the basis of intentions even when no consequences follow. Attempted murder is a crime. “Witches,” declared John Donne, “think sometimes that they kill when they do not, and are therefore as culpable as if they did.” “As for witches,” wrote Hobbes, “I think not that their witchcraft is any real power; but yet that they are justly punished, for the false belief they have that they can do such mischief, joined with their purpose to do it if they can.”

The belief-desire model, although indispensable, is fragile. The methods we use to impute mental states to other people do not always yield stable results. If we want to measure the height of a building, it does not matter whether we do it from the roof downward or from the ground upward.