Our problems in gaining the right conception of a rule is often connected with the distinction between description and prescription. I believe our problems in determining the modality of a rule stems from exactly these concepts, since our language is lacking in perspicuousness in distinguishing between the two following cases:
A rule considered from the point of view of a model (1) is a description, while a rule considered from point of view of praxis (2) is a prescription. And likewise a rule seems contingent from the point of view of the model, while it is seems necessary from the point of view of praxis.
(An recent example in point is the confusion of the famous philosopher of mind, Daniel Dennett, in ‘Philosophy as Naïve Anthropology’ in Bennett, Dennett, Hacker & Searle, Neuroscience and Philosophy (2008), p.73-97)
Contrast a description of the rules of chess within, say, a science of board games with the ruthless attention we pay the to rules of chess when we actually play chess. Within an empirical science of board games, such a description may be used to predict how people actually play chess, but it’s still only a contingently true description. Conversely, when we actually play chess, the rules are not descriptions, much less contingent, they are necessary prescriptions, they determine directly whether what we do is correct or not. In playing chess we apply the rules of chess, as it were, constitutively. They directly determine what is to count as X.
(In modern physics we call a relation between two physical quantities a ‘constitutive relation’, if the relation holds uniformly and yet doesn’t follow directly from the laws of physics. In grammar, too, we might speak of ‘constitutive rules’, if these establish a necessary relation between two entities, and yet doesn’t follow from the standard laws of logic.)
This distinction is as clear as anything, when it is just presented perspicuously: A linguist compiling a dictionary is making a description of how a word is used, while someone consulting a dictionary is using prescription of how a word is used.
But when we do social science we are so easily confused: We make our study of some field and say that this field conforms to these rules. E.g. that this field conforms to the axioms of microeconomics, that this field conforms to some Foucauldian apparatus, that this field conforms to the ruling ideology of management... But what are we charting by stating these rules? Some set of rules which fit these data OR some set of rules which guide the actors within the field?
A description fits something. A prescription guides someone. And the difference is vital.
Our failure to answer this last question puts the social sciences in an absurd situation. The situation is as absurd as if physicists wondered whether physical laws were descriptions or laws in the juridico-political sense. But it is not like that within physics: The orbital motion of the planets are described by the Keplerian laws, but certainly the planets do not comply with these laws.