Full Idea
Plato regards the substance of soul not as number but as being ordered by number.
Gist of Idea
Plato says the soul is ordered by number
Source
report of Plato (Timaeus [c.362 BCE]) by Plutarch - 68: Generation of the soul in 'Timaeus' 1023
Book Reference
Plutarch: 'Selected Essays and Dialogues', ed/tr. Russell,Donald [OUP 1993], p.223
A Reaction
This remark points towards Plato's esoteric doctrines, which are some sort of mathematical metaphysics. The idea that order and numbers are in some way connected is one of the most powerful in western civilization, with undeniable appeal.