Full Idea
If Herbert is disposed to not speak in history class (but not other subjects), and explanation of this is different from his avoidance of roller coaster rides, then these two dispositions are not special cases of a single character trait.
Gist of Idea
If a person's two acts of timidity have different explanations, they are not one character trait
Source
Gilbert Harman (Moral Philosophy meets social psychology [1999], 10.2)
Book Reference
Harman,Gilbert: 'Explaining Value and Other Essays' [OUP 2000], p.167
A Reaction
A basic Harman argument for denying the existence of character (and hence of virtues). I just say that character traits are more complex than his caricature of them. If I keep imagining disaster and humiliation for myself, that is a character trait.