As a field, though fertile, cannot yield a harvest without cultivation, no more can the mind without learning.

[Ut ager quamvis fertilis sine cultura fructuosus esse non potest, sic sine doctrina animus.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
Tusculan Disputations [Tusculanae Disputationes], Book 2, ch. 5 (2.5) / sec. 13 [Marcus] (45 BC) [tr. Peabody (1886)]
    (Source)

Often rendered in reverse order: "A mind without instruction can no more bear fruit than can a field, however fertile, without cultivation." (e.g., 1906). Original Latin. Alternate translations:

As a Field, though it be Fruitful, without Tillage cannot bring a good Crop, so the Soul without Learning.
[tr. Wase (1643)]

As the field naturally fruitful cannot produce a crop, without dressing, so neither can the mind, without improvement.
[tr. Main (1824)]

As the field, however fertile, cannot be fruitful without culture, so with the mind, without learning.
[tr. Otis (1839)]

As a field, although it may be naturally fruitful cannot produce a crop, without dressing, so neither can the mind, without education.
[tr. Yonge (1853)]

Just as a field however fertile cannot be fruitful without cultivation, neither can the soul without instruction.
[tr. Douglas (1990)]

Just as a field, however fertile, cannot be productive without cultivation, so the soul cannot be without teaching.
[tr. Davie (2017)]