Full Idea
Whoever understands some effect perfectly will also arrive at the knowledge of its cause.
Gist of Idea
Causes can be inferred from perfect knowledge of their effects
Source
Gottfried Leibniz (Meditatio de principio individui [1676], A6.3.490), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 2
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.60
A Reaction
This sounds highly improbable, given that you would have thought that there could be lots of ways to bring about the same effect. Predicting effects is rather more plausible. I suppose if you can record all the ripples in the pond before they fade...