“it is only when he has turned it over that he can be said to know it.”
Learning becomes knowledge, in this account, when material has been actively reconsidered and integrated rather than merely encountered or memorized.
Source details
The Art of Literature, section “On Thinking for Oneself,” T. Bailey Saunders translation; Project Gutenberg #10714. The preceding sentence says a person must first learn something before turning it over in the mind.