Full Idea
Good and evil are meaningless to things that have no soul.
Gist of Idea
To understand morality requires a soul
Source
Plato (Letter Seven [c.352 BCE], 334)
Book Reference
Plato: 'Phaedrus and Letters VII and VIII', ed/tr. Hamilton,Walter [Penguin 1973], p.127
A Reaction
That is presumably psuché, and hence includes plants. Soulless things can still function well, but obviously that is not 'meaningful' to them.