**Modal logic** is a form of logic which deals with sentences that are qualified by *modalities* such as *possibly,* *necessarily,* *contingently,* *actually,* *can,* *could,* *might,* etc. Unlike more traditional forms of first-order logic, which can only work with assertoric sentences (such as "Socrates is mortal," "This dog is a terrier," "All cats are reptiles," etc.), modal logic also deals with the logical relationships between problematic statements, such as "It's possible that it will rain on Thursday" or "I can choose to go to the movies tomorrow," and apodictic statements like "Every planet must have an orbit in the form of a conic section" or "if you add 2 and 2, the answer is necessarily 4."

The basic **modal operators** are usually given to be **possibility,** **actuality,** **necessity,** and **contingency.** A sentence is said to be **actual** if it is true; it is said to be **possible** if it might be true (whether it is actually true or actually false). A **necessary** statement is one which could not possibly be false; by contrast, a **contingent** statement is one that might be true *and also* might be false. (This is not the same, of course, as saying that it is a statement which might be *both* true and false; there are *no* statements of that sort.)

Table of contents |

2 Possible Worlds and the Interpretation of Modal Logic 3 Formal rules 4 External links |

Within modal logic, claims about **metaphysical modalities** (also known as **subjunctive modalities**) need to be distinguished from similar-sounding claims about **epistemic modalities**. For example, when a philosopher claims that Bigfoot possibly exists, he *probably* does not mean that "it's *possible that* Bigfoot exists--for all I know." Rather, he is making the *metaphysical* claim that "it's *possible for* Bigfoot to exist"--which is a substantive claim concerning ways the world could have been, with apparent ontological commitments.

On the other hand, suppose that someone asks you if 54 squared is 2926 and you stammer, "I don't know, I suppose it's possible." Here you are using an *epistemic* possibility--you are saying that "*For all I know,* it's *possible that* 54 squared is 2926." But you are almost surely not making the very hasty claim that it's *metaphysically possible* for 54 squared to be 2926--which is fortunate, since it turns out that 54 squared is 2916, and it's *metaphysically* impossible for it to have been otherwise.

Epistemic possibilities also bear on the actual world in a way that metaphysical possibilities do not. Metaphysical possibilities bear on ways the world *might have been,* but epistemic possibilities bear on the way the world *may be* (for all we know). Suppose, for example, that I want to know whether or not to take an umbrella before I leave. If you tell me "It's *possible that* it is raining outside"--in the sense of epistemic possibility--then that would weigh on whether or not I take the umbrella. But if you just tell me that "It's *possible for* it to rain outside"--in the sense of *metaphysical possibility*--then I am no better off for this bit of modal enlightenment.

The vast bulk of philosophical literature on modalities concerns *metaphysical* rather than *epistemic* modalities. (Indeed, most of it concerns the broadest sort of metaphysical modality--that is, bare logical possibility). This is not to say that metaphysical possibilities are more important to our everyday life than epistemic possibilities (consider the example of deciding whether or not to take an umbrella). It's just to that the priorities in philosophical investigations are rarely set by importance to everyday life--and *that* should be surprising to no-one.

In the most common interpretation of modal logic, one considers "all logically possible worlds". If a statement is true in all possible worlds, then it is a necessary truth. If a statement happens to be true in our world, but is not true in all possible worlds, then it is a contingent truth. A statement that is true in some possible world (not necessarily our own) is called a possible truth.

Whether this "possible worlds idiom" is the best way to interpret modal logic, and how literally this idiom can be taken, is a live issue for metaphysicians. For example, the possible worlds idiom which would translate the claim about Bigfoot as "There is some possible world in which Bigfoot exists". To maintain that Bigfoot's existence is possible, but not actual, one could say, "There is some possible world in which Bigfoot exists; but in the actual world, Bigfoot does not exist". But it is unclear what it is that making modal claims commits us to. Are we really alleging the existence of possible worlds, every bit as real as our actual world, just not actual? David Lewis infamously bit the bullet and said yes, possible worlds are as real as our own. This position is called "modal realism". Unsurprisingly, most philosophers are unwilling to sign on to this particular doctrine, seeking alternate ways to paraphrase away the apparent ontological commitments implied by our modal claims.

The concepts of necessity and possibility enjoy the following de Morganesque relationship:

- "It is
**not necessary that***X*" is equivalent to "It is**possible that not***X*. - "It is
**not possible that***X*" is equivalent to "It is**necessary that not***X*.

- Lp (necessarily p) has the same meaning as -M-p (not possible that not-p)
- Mp (possibly p) has the same meaning as -L-p (not necessarily not-p)

- Necessitation Rule: If p is a theorem of K, then so is Lp.
- Distribution Axiom: If L(p → q) then (Lp → Lq) (this is also known as axiom K)

- Lp → p (If it's necessary that p, then p is the case)

The system most commonly used today is **modal logic S5,** which robustly answers the questions by adding axioms which make all modal truths necessary: for example, if it's possible that p, then it's *necessarily* possible that p, and if it's necessary that p it's also necessary that it's necessary. This has the benefit that it fits well with our intuitions about the idiom of possible worlds: if P is true at all possible worlds, then it seems that there can be no possible world at which it is true that there is some possible world where P is false (for if there were such a world, then it would just be the case that P is *not* true at all possible worlds). Nevertheless, other systems of modal logic have been formulated, in part, because S5 may not be a good fit for every kind of metaphysical modality of interest to us. (And if so, that may mean that possible worlds talk isn't a good fit for these kinds of modality either.)

- Stanford Encyclopedia of Philosophy entry
- A discussion of modal logic by John McCarthy