You have to study a long time to know anything at all.

[Il fout avoir beaucoup étudié pour savoir peu.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Pensées Diverses [Assorted Thoughts] (1720-1755) [ed. Guterman (1963)]
    (Source)

(Source (French)). Other translations:

In order to know nothing, it is necessary that a man should have studied a great deal.
[tr. Dale (1882)]

It is necessary to study much in order to know little.
[E.g.]

You have to study a great deal to know a little.
[E.g.]