Extended modal realism is a metaphysical theory developed by Takashi Yagisawa.
There is only one universe encompassing everything that is real in the widest sense: the actual, the possible, and the impossible.
Modal statements, like "Hillary Clinton could have won the 2016 US election" or "it is necessary that cows are animals", are part of everyday language.
Modal realists, by contrast, state that possibility is an irreducible aspect of reality besides actuality.
According to modal realists like David Lewis, possible worlds are isolated entities.
This could be expressed by saying that the green color is located at some spatial indices occupied by the tree while the brown color is located at other spatial indices occupied by the tree.
[9] The tree as a whole can be pictured as a space-time worm: it is made up of various parts located at different spatial and temporal indices.
[4]: 41 [10] Another way to express these ideas is to say that space, time, and modality are relativizers of qualities like colors.
Whether a modal statement about an object is true depends on its parts belonging to non-actual worlds.
The universe is the vast object extending in the dimensions of space, time, and modality.
For example, various theorists have used Occam's razor to argue that we should not assume that possible worlds exist since "entities should not be multiplied beyond necessity".
As John Divers points out, the term "reality" may be useless if we allow possible as well as impossible objects into our ontology.
This consequence would turn metaphysics, when understood as the science of reality, into a trivial enterprise.
Yagisawa rejects this line of argument by holding that it is based on a false inference.
He suggests the strict usage of a modally tensed language to avoid this problem.