“A man who does not think for himself does not think at all.”

Maxim 584
Sentences, The Moral Sayings of Publius Syrus, a Roman Slave

Source: The Liberty of Man, Woman and Child (1877)
Context: Only a few years ago there was a great awakening of the human mind. Men began to inquire by what right a crowned robber made them work for him? The man who asked this question was called a traitor. Others asked by what right does a robed hypocrite rule my thought? Such men were called infidels. The priest said, and the king said, where is this spirit of investigation to stop? They said then and they say now, that it is dangerous for man to be free. I deny it. Out on the intellectual sea there is room enough for every sail. In the intellectual air there is space enough for every wing.
The man who does not do his own thinking is a slave, and is a traitor to himself and to his fellow-men.

Source: The Autobiography of Eleanor Roosevelt

IV, 18
Source: Meditations (c. 121–180 AD), Book IV

Part I. Généralités (Generalities), Chapter I. Prolégomènes (Prolegomena).
Treatise on Elegant Life (1830)
Original: (fr) Or les trois classes d'être créés par les mœurs sont :
L'homme qui travaille ;
L'homme qui pense ;
L'homme qui ne fait rien.

"On Eating and Drinking".
Source: Idle Thoughts of an Idle Fellow (1886)
Context: Foolish people — when I say "foolish people" in this contemptuous way I mean people who entertain different opinions to mine. If there is one person I do despise more than another, it is the man who does not think exactly the same on all topics as I do.

Source: The Virtue of Selfishness: A New Concept of Egoism

“Schopenhauer as educator,” § 3.1, R. Hollingdale, trans. (1983), p. 127
Untimely Meditations (1876)