Abstract:

The necessity involved in modal olgic is manifold. The logical laws as theorems in a logical system embody logical necessity, while the necessity operators in modal predicate formulas which reflect "wodality de re" mainly embody factual necessity. The rnodal axioms in a modal logical system describe its logical characteristic of necessity from another respect. The definition of necessity is closely related to a possible world. When analyzing essential attributes of things in the real world, we should restrict the range of a possible world and abandon non-real situations of the real world.

Key words: modal logic, logical necessity, factual necessity, possible world, essence