Full Idea
Plato points to the truth about the highest good when he says that it cannot be expressed in words, but rather comes from familiarity - like a flash from the blue, imprinting itself upon the soul.
Gist of Idea
The good cannot be expressed in words, but imprints itself upon the soul
Source
report of Plato (The Republic [c.371 BCE]) by Celsus - On the True Doctrine (Against Christians) VII
Book Reference
Celsus: 'On the True Doctrine (Against the Christians)', ed/tr. Hoffmann,R.Joseph [OUP 1987], p.91
A Reaction
It is reasonable to be drawn to something inexpressible, such as an appealing piece of music, but not good philosophy to build a system around something so obscure.