S4 and S5 are the beginning of what is now called normal modal logic.

There are then at least three modal logics that we can develop.

This holds if a world may have more than one successor, which is true for most modal logic.

The expansion rules depend on the particular modal logic used.

These possible worlds arise in the field of modal logic.

Van Benthem is known for his research in the area of modal logic.

Lewis' own development of multiple modal logics is a case in point.

This chapter and the subsequent ones are at the origin of modal logic.

A good introduction to the varied interpretations of modal logic.

In the most common semantics for modal logic, many "possible worlds" are considered.