 What animates crowds is mental contagion, which is the psychological mechanism whereby the mental states of one individual pass (often unconsciously) to another individual. In crowds, individuals form mental states on the basis of others’ mental states, but the induced states are entertained without a reason. So, for instance, the warranted joy of a subject x may infect individual y who elicits an emotional episode of the same kind, but without having a reason for it. This is what makes crowds different from societies: in societies, too, individuals form mental states on the basis of others’ mental states, but here the individuals take the other’s mental state to be a reason for them to entertain their own states. For instance, agent x might form the intention to act with agent y (and vice versa) because x and y come to know that each of them pursues the same goal.