IMMANUEL KANT. An Answer to the Question: What is Enlightenment? (1784)

Enlightenment is man's emergence from his self-imposed immaturity. Immaturity is the inability to use one's understanding without guidance from another [...] Sapere Aude! [dare to know] "Have courage to use your own understanding!"--that is the motto of enlightenment.

Laziness and cowardice are the reasons why so great a proportion of men, long after nature has released them from alien guidance [...], nonetheless gladly remain in lifelong immaturity, and why it is so easy for others to establish themselves as their guardians. It is so easy to be immature. If I have a book to serve as my understanding, a pastor to serve as my conscience, a physician to determine my diet for me, and so on, I need not exert myself at all. I need not think, if only I can pay: others will readily undertake the irksome work for me.

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Nothing is required for this enlightenment, however, except freedom; and the freedom in question is the least harmful of all, namely, the freedom to use reason publicly in all matters.

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The public use of one's reason must always be free, and it alone can bring about enlightenment among mankind; the private use of reason may, however, often be very narrowly restricted, without otherwise hindering the progress of enlightenment. By the public use of one's own reason I understand the use that anyone as a scholar makes of reason before the entire literate world. I call the private use of reason that which a person may make in a civic post or office that has been entrusted to him. [...] Here one certainly must not argue, instead one must obey.

Source: http://www.english.upenn.edu/~mgamer/Etexts/kant.html