Full Idea
It is a Humean uniformity that no race of ravens is white-feathered. Hence, if the Naive Regularity analysis of law is correct, it is a law that no race of ravens is white-feathered, that is, such a race is physically impossible. A most unwelcome result.
Gist of Idea
A naive regularity view says if it never occurs then it is impossible
Source
David M. Armstrong (What is a Law of Nature? [1983], 02.6)
Book Reference
Armstrong,D.M.: 'What is a Law of Nature?' [CUP 1985], p.18
A Reaction
Chapters 2-4 of Armstrong are a storming attack on the regularity view of laws of nature, and this idea is particularly nice. Laws must refer to what could happen, not what happens to happen.