Full Idea
For Plato, an acceptable explanation is one such that there is no possibility of there being the opposite explanation at all, and he thought that only explanations in terms of the Forms, but never physical explanations, could meet this requirement.
Gist of Idea
A good explanation totally rules out the opposite explanation (so Forms are required)
Source
report of Plato (works [c.375 BCE]) by David-Hillel Ruben - Explaining Explanation Ch 2
Book Reference
Ruben,David-Hillel: 'Explaining Explanation' [Routledge 1990], p.58
A Reaction
[Republic 436c is cited]