The last two decades have witnessed a basic change in the philosopher's working model for capacities. The change has been from a stimulus-response model to a fine-structure model. The stimulus-response model was an attempt to understand capacity propositions by means of conditional propositions in which the antecedents expressed operations in certain circumstances and the consequents expressed realizations of the capacities in question. No attempt is made on this model to relate such conditional propositions to components of the entities with the capacities. An entity with a capacity is just a black box.

Now the conditional proposition to which a capacity proposition is related on the stimulus-response model is a modal conditional. It expresses the idea that the behavior characteristic of the capacity must follow upon those operations in those circumstances for an entity like one with the capacity.