sine doctrina animus

ut ager quamvis fertilis sine cultura fructuosus esse non potest, sic sine doctrina animus

Just as a field, however fertile, is not able to be fruitful without cultivation, so it is for the mind without instruction.

Cicero, Tusculan Disputations, II.5.13

[My attention was drawn to this quote by the article The Romans Gave us more than Just Latin; the Latin source comes from LatinLanguage.us.]