Full Idea
A good reason for P is not necessarily an explanation of P. The presence of smoke is a good reason for thinking that fire is present. But it is not an explanation of the presence of fire.
Gist of Idea
A good reason for something (the smoke) is not an explanation of it (the fire)
Source
David M. Armstrong (What is a Law of Nature? [1983], 04.2)
Book Reference
Armstrong,D.M.: 'What is a Law of Nature?' [CUP 1985], p.40
A Reaction
This may be an equivocation on 'the reason for'. Smoke is a reason for thinking there is a fire, but no one would propose it as a reason for the fire. If the reason for the fire was arson, that would seem to explain it as well.