Another example may help bring out the problem. Suppose a mad scientist could rewire my brain so that I became a crazed serial killer. It seems absurd to say that I already have the disposition to become a crazed serial killer "if rewired". The antecedent condition is one that changes who I am - my character and dispositions - so the counterfactual doesn't really reflect at all on my actual character and dispositions. (Cf. the conditional fallacy.)
I guess the key issue here is whether my current state contains the "causal basis" of my counterfactual behaviour, or whether this is something that's newly introduced by the antecedent condition. Drastic neural rewiring alters the causal basis of my behaviour. But all thought and experience involves some degree of neural rewiring - our brain changes, that's how we learn! So how can we draw any strict line here, between changes that merely draw on our existing dispositions, and changes that introduce whole new ones? Our terms suggest a difference in kind, when the neurobiology may merely differ in degree.
The above discussion seems lacking in conceptual clarity. There must be a better way to understand these issues. Any suggestions? (Quick thought: perhaps dispositions should be understood more exclusively as general tendencies, manifest in a much broader range of behaviours. Then we may discard the above cases as too isolated and specific to justify invoking 'dispositions'.)
Similar issues arise for dispositional beliefs. Peter distinguishes between dormant actual beliefs ("my name is Peter") and merely derivable beliefs ("25 - 11 = 14"). He suggests that the difference consists in whether the proposition is "directly integrated into the person’s mental operations", and so "immediately available when needed". He adds:
It is natural to say that plain beliefs are always in mind, although usually unconscious, but derived beliefs are not usually present in any way, except when they are consciously derived from plain beliefs.
This sounds like the above distinction between owned (internal) and disowned (external) dispositions; those that are already present in the agent, and those that aren't (but would be given the right stimulus). But aren't integration and availability likewise matters of degree?