Norms and Necessity, by Amie Thomasson

Extract

Imagine you’re teaching someone how to play chess. You might start by saying ‘White must move first’, where the word ‘must’ is used to convey a rule. You would have said basically the same thing if you had used the imperative ‘If you’re white, then move first’. And since imperatives prescribe rather than describe, it is natural to think that using a must-statement to convey a rule is importantly different from describing something. More specifically, at least when you are teaching someone a game, the word ‘must’ does not seem to describe what always happens or even to describe what the rule book says but rather to affirm or institute rules.

This may be the core function of deontic necessity modals such as ‘must’, but not every deontic use of this word can be translated into an imperative. For example, your pupil might reason ‘If white must move first, then black must move second’. Here, the pupil has conditionalized on the rule, and in order to do so, the rule needs to be stated in declarative form to work grammatically as the antecedent of a conditional. Nevertheless, the resulting statement still seems to be something other than a description; the pupil has derived what we might call a normative consequence from the original rule.