Full Idea
The manifestations of a disposition have the particularly mysterious property (metaphysically speaking) that they need not exist - which makes them rather like intentional objects.
Gist of Idea
The manifestations of a disposition need never actually exist
Source
David M. Armstrong (Pref to new 'Materialist Theory' [1992], p.xvii)
Book Reference
Armstrong,D.M.: 'A Materialist Theory of Mind' [Routledge 1993], p.-7
A Reaction
His example is a brittle glass which never shatters. This problem seems to require the mention of conditional and counterfactual statements in the description of the actual world, which rather increases the workload for philosophers.