Full Idea
If there be posited different ultimate ends, whose causes are opposed to each other, then there will be truly good actions likewise opposed to each other, which is impossible.
Gist of Idea
If there are different ultimate goods, there will be conflicting good actions, which is impossible
Source
Richard Cumberland (De Legibus Naturae [1672], Ch.V.XVI)
Book Reference
'British Moralists 1650-1800 Vol. 1', ed/tr. Raphael,D.D. [Hackett 1991], p.99
A Reaction
A very interesting argument for there being one good rather than many, and an argument which I don't recall in any surviving Greek text. A response might be to distinguish between what is 'right' and what is 'good'. See David Ross.