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# Modal Logic

Disciplines Argument > Types of reasoning > Modal Logic

## Description

Describe things in terms of possibility and necessity. Also explore how they intertwine.

For possibility, do not state things in terms of absolute truth, but say how likely things may be. Use words like seldom, often, probable, possibly, could, unusual.

For necessity, talk about how necessary something is. Thus use words like can, may, should, ought, must, have to.

Talking about how true or necessary something is gives you more potential in arguments as you now have an analog continuity of alternatives, rather than the black-and-white binary decision of simply whether something is true or false, necessary or unnecessary.

## Example

 Say this Not this The door might be open. The door is open. You must do it. You do it. They could come here. They will come here.

## Discussion

Traditional logic is based on extension, in that the truth of the logic is found within the supporting statements. Modal logic is based on intention, in that truth is where you find it, and that the reality of many situations is that it is impossible to determine exact truth.

Thus:

• A sentence is possible if it might be true (or might be false).
• A sentence is necessary if it must be true (and cannot possibly be false).
• A sentence is contingent if it is not necessarily true. (a contingent truth is true in the given case, but might not have been true).

Necessity and possibility have aspects of a Boolean relationship in that:

It is not necessary that X is true  =  It is possible that X is not true

It is not possible that X is true = It is necessary that X is not true

The modalities of possibility and necessity are also known as alethic modalities.

Deontic logic is the specific logic about duty, where necessity is has a moral quality to it.

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