You think of reality as being what is, as against what is not. And what is possible is a mental construct needed for optimised modeling of what is in time - time also being an aspect of what is. This kind of implicit account is difficult to think through all the way, but nevertheless is believed, even at the cost of assuming multiple worlds. Alternatively you might see the possible as the also integral to reality and not as a deficient approximation to the sole reality that is, and if you take this position, allowing a third valence to is and is not, then many problems of matter and mind disappear. In fact the is and is not end up dissolved into the all-embracing modality of the possible, which now requires a new name.