Full Idea
If a thing possesses any kind of intrinsic value in a certain degree, anything exactly like it must in all circumstances possess it in the same degree.
Gist of Idea
Identical objects must have identical value
Source
W. David Ross (The Right and the Good [1930], §IV)
Book Reference
Ross,W.David: 'The Right and the Good' [OUP 1930], p.115
A Reaction
This is the earlier notion of supervenience in philosophy, before it was applied to the mind. So a perfect duplication of the Mona Lisa will be worth as much as the original? A perfect clone of your partner is as good as the original?