This morning I’ve been thinking about dispositions that cannot be manifested: that is, dispositions to 
under circumstances C, where either -ing or circumstances C are metaphysically impossible.
One thing I’m interested in is whether there are any such dispositions. Another is whether anything has such a disposition. Prima facie, there are some reasons to answer yes to both questions. I think, for instance, that I have a disposition to be puzzled when presented with a round square object.
In response to this suggestion, however, Daniel pointed out that a certain amount of coarse-graining about dispositions would enable us to accommodate that disposition without believing in dispositions which cannot be manifested. My disposition to be puzzled when presented with a round square object may be identical to my disposition to be puzzled when presented with an interesting and surprising object that I didn’t think existed, and this disposition can of course be manifested.
Lewis’s counterfactual account of dispositions in ‘Finkish Dispositions’, combined with his view that counterpossible conditionals are trivially true, delivers that everything has every disposition to in circumstances C for impossible C. But this does not by itself entail that there are any dispositions which cannot be manifested, since these trivial dispositions may for all we’ve said so far be identical to more familiar, manifestable, ones.
Nevertheless, for those of us inclined to be abundant with our dispositions, I think there is some reason to believe in unmanifestable dispositions (and instantiations thereof). And I don’t see any special reason why there shouldn’t be such things, given that dispositions don’t need to be manifested in order to be instantiated.
