Rules as contingent rules of inference
The set of rules of inference is not fixed in the logic (as ‘ordinary’, tautologous rules of inference, such as Modus ponens) but is part of the domain theory.
? Modus ponens: p ? q, p / q
The rule that, if John has stolen a bike, then he is a thief, becomes a contingent rule of inference:
John has stolen a bike / John is a thief