 The debate between possibilists and actualists is at root ontological. It is not—fundamentally, at least—a debate about meaning, or the proper linguistic primitives of our modal discourse, or the model theory of certain formal languages, or the permissibility of certain inferences. It is a disagreement over what there is, about the kinds of things that reality includes. Characterizing the nature of the debate precisely, however, is challenging. As noted in the preceding introduction, the debate centers around the question of whether, in addition to such things as you and me, reality includes possibilia, such as Bergoglio’s merely possible children, the children he does not actually have but could have had if only things had gone rather differently. Clearly, if there are such things in some sense, they differ from us rather dramatically: a merely possible child is not actually anyone’s child and indeed, we are inclined to say, does not actually exist at all and, hence, is not actually conscious but only could have been, does not actually have a body but only could have had one, and so on. As we will see in more detail in the following section, in many historical and more contemporary discussions, this alleged difference is characterized in terms of distinct ways, or modes, of being, and it will be useful, for now, to continue to frame the debate in these terms. Specifically, on this bi-modal conception of the debate, on the one hand, there is the more substantial and robust mode of actuality (or, often, existence) that you and I enjoy. And, on the other hand, there is the more rarefied mode (sometimes called subsistence) exhibited by things that fail to be  actual.[4]  While (as we will see) there has been some disagreement over the location of abstract objects like the natural numbers in this scheme, what uniquely distinguishes possibilia on the bi-modal conception is that, for the possibilist, they are contingently non-actual: