What are the senses, such that synesthesia can be characterized as a “union” of them? This chapter explores the relationship between the neuropsychological phenomenon of synesthesia and our understanding of the senses, particularly how many there are. After giving a brief introduction to our understanding of the senses and synesthesia, I then present three different accounts of the nature of the senses. Each of these is derived from different aspects of our commonsense understanding of the senses, including the nature of the sensory organ, of the feature of the world sensed and of the  characteristic conscious experience unique to the operation of each sense. I argue that the facts of synesthesia play out differently in reaction to each of these different accounts, but that ultimately, the phenomenon does not let us distinguish between them in terms of how many senses they would identify in a synesthetic. Further, the facts of synesthesia do not show that one account is correct or that the others are wrong. However, the differences in the sorts of considerations needed to make sense of synesthesia on these different accounts does demonstrate the real differences that exist between them.