 What distinguishes actuality from all other realms? The leading answer is due to David Lewis, who is the proponent of the best-known version of possibilist realism, namely, modal counterpart theory. Lewis’s view on actuality is known as the indexical theory of actuality (Lewis 1970). The basic idea is that actuality for us is the realm which includes us, and more generally, actuality for x is the realm which includes x. But there are many different realms which include us: this room, this building, this town, this continent, this planet, this galaxy, this minute, this hour, this day, this month, this year, this century, etc. So, to fix our actuality as a unique realm which includes us, Lewis takes the largest spatiotemporal whole which includes us. More precisely, actuality for us is the maximal spatiotemporally related whole of which we are (mereological) part. In general, actuality for x is the maximal spatiotemporally related whole of which x is part. For anything to exist non-actually-for-x but possibly is for it to be part of some realm outside actuality for x, that is, to be part of some maximal spatiotemporally related whole of which x is not part. Note that this is a reductionist view of existence, both actual and non-actual possible. Existence is first relativized to a maximal spatiotemporally related whole, and then existence in such a whole is defined reductively in terms of (mereological) parthood. Lewis characterizes possible worlds as maximal spatiotemporally related wholes. Actuality is the actual world, and all other maximal spatiotemporally related wholes are non-actual possible worlds.