 Suppose my friend recently suffered a devastating loss and I realize she is feeling sad. The associative principles transmit force and vivacity from my vivid awareness of myself to my idea of my friend’s sadness. Since for Hume the difference between impressions and ideas is that impressions are more lively and vivacious than ideas, if an idea of a passion is sufficiently enlivened, it becomes the very passion itself. I now feel sad too, but not quite as strongly as my friend.