The soul languishing in ...

The soul languishing in obscurity contracts a kind of rust, or abandons itself to the chimera of presumption; for it is natural for it to acquire something, even when separated from any one.
 Quintilian

Quotes from the same author

Though ambition may be a fault in itself, it is often the mother of virtues.
 Quintilian
One thing, however, I must premise, that without the assistance of natural capacity, rules and precepts are of no efficacy.
 Quintilian
Prune what is turgid, elevate what is commonplace, arrange what is disorderly, introduce rhythm where the language is harsh, modify where it is too absolute.
 Quintilian
Men of quality are in the wrong to undervalue, as they often do, the practise of a fair and quick hand in writing; for it is no immaterial accomplishment.
 Quintilian
Satiety is a neighbor to continued pleasures. [Lat., Continuis voluptatibus vicina satietas.]
 Quintilian