Modal realism is the view propounded by David Kellogg Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours." It is based on the following tenets: possible worlds exist; possible worlds are not different in kind from the actual world; possible worlds are irreducible ; the term "actual" in "actual world" is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now".