Unconditional freedom of thought
The excesses of an unbridled intellect, which unfailingly end in the oppression of the untutored multitude, are no less rightly controlled by the authority of the law than are the injuries inflicted by violence upon the weak. And this all the more surely, because by far the greater part of the community is either absolutely unable, or able only with great difficulty, to escape from illusions and deceitful subtleties, especially such as flatter the passions. If unbridled license of speech and of writing be granted to all, nothing will remain sacred and inviolate; even the highest and truest mandates of natures, justly held to be the common and noblest heritage of the human race, will not be spared. Thus, truth being gradually obscured by darkness, pernicious and manifold error, as too often happens, will easily prevail. Thus, too, license will gain what liberty loses; for liberty will ever be more free and secure in proportion as license is kept in fuller restraint. In regard, however, to all matter of opinion which God leaves to man's free discussion, full liberty of thought and of speech is naturally within the right of everyone; for such liberty never leads men to suppress the truth, but often to discover it and make it known. From what has been said it follows that it is quite unlawful to demand, to defend, or to grant unconditional freedom of thought, of speech, or writing, or of worship, as if these were so many rights given by nature to man. For, if nature had really granted them, it would be lawful to refuse obedience to God, and there would be no restraint on human liberty. It likewise follows that freedom in these things may be tolerated wherever there is just cause, but only with such moderation as will prevent its degenerating into license and excess. (Papal Writings, Libertas, 1888).