Truth that has been merely learned is like an artificial limb, a false tooth, a waxen nose; at best, like a nose made out of another’s flesh; it adheres to us only because it is put on. But truth acquired by thinking of our own is like a natural limb; it alone really belongs to us. This is the fundamental difference between the thinker and the mere man of learning.
 
[Hingegen klebt die bloß erlernte Wahrheit uns nur an, wie ein angeseßtes Glied, ein falscher Zahn, eine wächserne Nase, oder höchstens wie eine rhinoplastische aus fremdem Fleische. Die durch eigenes Denken erworbene Wahrheit aber gleicht dem natürlichen Gliede: fie allein gehört uns wirklich an. Darauf beruht der Unterschied zwischen dem Denker und dem bloßen Gelehrten.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 2, ch. 22 “On Thinking for Oneself [Selbstdenken],” § 260 (1851) [tr. Saunders (1890)]
    (Source)

Source (German). Alternate translations:

Truth that has been merely learned adheres to us like an artificial limb, a false tooth, a waxen nose, or at best like one made out of another's flesh; truth which is acquired by thinking for oneself is like a natural member: it alone really belongs to us. Here we touch upon the difference between the thinking man and the mere man of learning.
[tr. Dircks (1897)]

Truth that has merely been learnt adheres to us only as an artificial limb, a false tooth, a was nose does, or at most like transplanted skin; but a truth won by thinking for ourself is like a natural limb: it alone really belongs to us. This is what determines the difference between a thinker and a mere scholar.
[tr. Hollingdale (1970)]

... other hand, the truth acquired through our own thinking is like the natural limb; it alone really belongs to us. On this rests the distinction between the thinker and the mere scholar.
[tr. Payne (1974)]