This brief clip discusses modality and the notion of possible worlds in metaphysics. Gottfried Leibniz came up with the notion of possible worlds as a way of emphasizing the privileged status that the actual world holds over other potential states of affairs because God consciously created it this way for mankind. Leibniz is famous for stating that this is "the best of all possible worlds."
Next, the clip discusses our modal intuitions concerning "possibility" and "necessity," and how they can be best understood within the context of possible worlds.
"Contingent propositions" are those that are true in some possible worlds and false in others. For example: "Richard Nixon became President in 1969" is contingently true and "Hubert Humphrey became President in 1969" is contingently false. While "necessary propositions" are those that are true in all possible worlds. For example: "2+2=4" and "All bachelors are unmarried."
This leads into a discussion of David Lewis and his claim known as "modal realism;" the view that all logically possible worlds exist in the same sense that the actual world exists. In other words, that these other possible worlds are "really out there."
And finally, the video mentions the striking similarities between Lewis' modal realism and the "Many-Worlds Interpretation" of quantum mechanics.
