This article considers a central question in the philosophy of emotion: what is an (instance of) emotion? This is a highly controversial question, which has attracted numerous answers. I argue that a good answer to this question may prove very hard to find. The difficulty, I suggest, can be traced back to three features of emotional phenomena: their diversity, their complexity and their coherence. I end by suggesting that we should not be disturbed by this result, as we do not need to know what an instance of emotion is in order to investigate the topic of emotion.