There are five fingers on your hand, five points to a pentagram, and five circles in the Olympic logo. But is there such a thing as a number called five? And if so, what kind of a thing is it? As in our Chap. 2 discussion of one: is five an entity, or an activity, or both? We can say five is a number, but that is just to put the question in more general terms. What is a number? More generally, do numbers really exist, or are they just a convenient fiction we have invented?