Full Idea
'Why does that quail in the garden bob its head up and down in that funny way whenever it walks?' …'Because they all do'.
Gist of Idea
You can't explain one quail's behaviour by just saying that all quails do it
Source
Nancy Cartwright (How the Laws of Physics Lie [1983], 3.5)
Book Reference
Cartwright,Nancy: 'How the Laws of Physics Lie' [OUP 2002], p.70
A Reaction
She cites this as an old complaint against the covering-law model of explanation. It captures beautifully the basic error of the approach. We want to know 'why', rather than just have a description of the pattern. 'They all do' is useful information.