The state of affairs which obtain in the world, is reality. Whereas the combinations of the state of affairs that do not obtain are not the real world. Niether are they even other possible worlds. Prior begins with the analogy of a world of Greek myth-makers versus the real world. To say that ``there are possible worlds in which centaurs exist,'' implies simply that ``it could be that there are centaurs.'' If you were going to say that ``there are horses,'' then you would just say that without any qualifiers (of a temporal or spacial nature.)
Hence, to say that ``I am writing this essay now,'' is stating a set of affairs that are currently true. The present, as a ``special'' region of space separating it from the future or the past, makes no sense. Instead the present is an absence of a qualifying prefix of a temporal nature. Using the previous similar fallacious stream of thought, one might be tempted to say that time is, in its own reality; its own dimension. However, we shall cover that later in this essay.