Full Idea
If intelligence and true opinion are different, then the forms must exist, but if they are the same, then what our senses perceive must be the most certain reality.
Gist of Idea
For knowledge and true opinion to be different there must be Forms; otherwise we are just stuck with sensations
Source
Plato (Timaeus [c.362 BCE], 51d)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Lee,Desmond [Penguin 1971], p.71