Tuesday, January 28, 2014

Terminology Tuesday: Modal Logic

Modal Logic: Study of reasoning about what must or might be the case, as well as what merely happens to be the case. The formalization of modal logic for the propositional calculus introduces special operators designating necessity and possibility (L and M). “It is necessary that p” (Lp) is interpreted to mean that p must be true in all possible worlds, and “It is possible that p” (Mp) that p is true in at least one possible world. On these interpretations,
Lp º ~M~p    is tautologous.1

See also: Modal Logic: An Introduction by Brian F. Chellas

1. Philosophy Pages.


  1. Nate December 14, 2010

    I don't understand this at all.

  2. Nick January 28, 2014

    An example would be great. I don't understand this fully.

  3. Nick January 28, 2014

    An example would be great to understand this better.

  4. John January 28, 2014

    The tautology on the bottom helps when you say it out loud: "It's necessary that p means that it's not possible that not p." In no possible worlds can not p exist, therefore, p is necessary, or, p exists all the time everywhere.

  5. BSZ January 29, 2014

    Anyone interested on a good summary of logic is suggested to purchase: "The Little Logic Book". The book is a collaboration of 4 philosophers from Calvin College. I think many here at Apologetics 315 will benefit from it.

    In Christ,