Full Idea
The dilemma is that to give the ultimate source of any necessity, we must either appeal to something which could not have been otherwise (i.e. is itself necessary), or advert to something which could have been otherwise (i.e. is itself merely contingent).
Gist of Idea
Explanation of necessity must rest on something necessary or something contingent
Source
Bob Hale (The Source of Necessity [2002], p.301)
Book Reference
-: 'Philosophical Perspectives' [-], p.301
A Reaction
[Hale is summarising Blackburn's view, and going on to disagree with it] Hale looks for a third way, but Blackburn seems to face us with quite a plausible dilemma.