How we change what others think, feel, believe and do
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.
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.
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.