 The way Hume uses the idea that the associative principles transmit force and vivacity in his explanation of sympathy is parallel to the way he uses it in his explanation of causal inference. In the case of causal inference, if we have an impression of an effect (smoke), the associative principles give rise not only to the idea of its cause (fire), but they also transmit some of the impression’s force and vivacity to the idea of its cause, so that we come to believe that fire is the cause of the smoke. A belief is an idea that is so lively that it is like an impression, and influences us in the way impressions do. Similarly, my lively awareness of myself enlivens by association my idea of my friend’s sadness. But the result in the case of sympathy is even stronger: when an idea of a passion is sufficiently enlivened, it becomes the very passion itself.