Arthur Schopenhauer Quote

As the biggest library if it is in disorder is not as useful as a small but well-arranged one, so you may accumulate a vast amount of knowledge but it will be of far less value to you than a much smaller amount if you have not thought it over for yourself; because only through ordering what you know by comparing every truth with every other truth can you take complete possession of your knowledge and get it into your power. You can think about only what you know, so you ought to learn something; on the other hand, you can know only what you have thought about.

Vol. 2, Ch. 22, § 257 "On Thinking for Yourself" as translated in Essays and Aphorisms (1970) as translated by R. J. Hollingdale - Parerga and Paralipomena (1851) - Counsels and Maxims