Possible worlds

Grass is green. But it could have been blue. Two plus two is four, and it could not have been anything else. A square circle cannot exist at all. We make claims like these constantly. They are about modality: what is possible, necessary or impossible.

Gottfried Leibniz, and later David Lewis, gave a helpful picture. Imagine every complete way the world could have been. Each is a possible world. Ours is the actual world.

Then modal words become simple: a statement is possible if it is true in at least one possible world. It is necessary if it is true in all of them. It is impossible if it is true in none.

Lewis thought these other worlds are just as real as ours, only far away in no direction at all. Most philosophers treat them as a useful model, like a map of possibilities, rather than real places.