Arthur Schopenhauer book Parerga and Paralipomena
Vol. 2, Ch. 22, § 261
Variant translations:
Reading is thinking with some one else's head instead of one's own.
As translated by T. Bailey Saunders
Reading is equivalent to thinking with someone else’s head instead of with one’s own.
Parerga and Paralipomena (1851), Counsels and Maxims