Full Idea
We don't have, nor should we hope for, any mark of reality in phenomena, but the fact that they agree with one another and with eternal truths.
Gist of Idea
The only indications of reality are agreement among phenomena, and their agreement with necessities
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1706.01.19)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.186
A Reaction
Elsewhere he says that divisions in appearance imply divisions in matter. Now he adds two further arguments in favour of realism, but admits that nothing conclusive is available. Quite right.