Milton's Areopagitica was a response to a licensing law passed by Parliament in 1643. This law required, essentially, that books be licensed, or approved by a government censor, before they were published. Milton argued forcefully and eloquently against this law, and for freedom of the press in general. This was because, he argued, the free exchange of ideas was healthy and conducive to the virtue that he felt was essential to maintaining a free society. If ideas were evil, blasphemous, or generally in error, then they would be revealed to be so by free and open disputation. In fact, Milton observed, a person's virtue was weakened when it was not challenged. In ringing and memorable language, he asserted the power of books, and of ideas, which were meant to be examined with God-given reason:
As good almost kill a man as kill a good book: who kills a man kills a reasonable creature, God's image; but he who destroys a good book, kills reason itself, kills the image of God, as it were, in the eye.
It should be noted that Milton was not arguing for free speech in the all-encompassing sense that we understand it today. He and his contemporaries accepted that individuals could be punished for the publication of certain ideas after the fact. But what troubled Milton was the possibility that censorship and what we would today call "prior restraint" would weaken the public virtue and make intellectual inquiry impossible. For its style as well as the ideas it expresses, this pamphlet remains a seminal document in the Anglo-American tradition of civil liberties.