Philosophical discussion of the notion of existence, or being, has centered on two main problems that have not always been very clearly distinguished. First, there is the problem of what we are to say about the existence of fictitious objects, such as centaurs, dragons, and Pegasus; second, there is the problem of what we are to say about the existence of abstract objects, such as qualities, relations, and numbers. Both problems have tempted philosophers to say that there are inferior sorts of existence as well as the ordinary straightforward sort, and they therefore often suggest that we use the word being to cover both kinds but restrict "existence" to "being" of the common, nonfictitious, nonabstract sort. (Sometimes the term reality is proposed for "existence" or for "being.") The problems of fiction and abstraction are different, however, for there are both real and fictitious abstractions. For example, the integer...