At the heart of this new optimistic view of the possibility of knowledge lies the doctrine that truth is manifest. Truth may perhaps be veiled. But it may reveal itself. And if it does not reveal itself, it may be revealed by us. Removing the veil may not be easy. But once the naked truth stands revealed before our eyes, we have the power to see it, to distinguish it from falsehood, and to know that it is truth.
The birth of modern science and modern technology was inspired by this optimistic epistemology whose main spokesmen were Bacon and Descartes. They taught that there was no need for any man to appeal to authority in matters of truth because each man carried the sources of knowledge in himself; either in his power of sense-perception which he may use for the careful observation of nature, or in his power of intellectual intuition which he may use to distinguish truth from falsehood by refusing to accept any idea which is not clearly and distinctly perceived by the intellect. Man can know: thus he can be free. This is the formula which explains the link between epistemological optimism and the ideas of liberalism.
This link is paralleled by the opposite link. Disbelief in the power of human reason, in man’s power to discern the truth, is almost invariably linked with distrust of man. Thus epistemological pessimism is linked, historically, with a doctrine of human depravity, and it tends to lead to the demand for the establishment of powerful traditions and the entrenchment of a powerful authority which would save man from his folly and his wickedness. (There is a striking sketch of this theory of authoritarianism, and a picture of the burden carried by those in authority, in the story of The Grand Inquisitor in Dostoievsky’s The Brothers Karamazov.)
—Karl Popper, Conjectures and Refutations: The Growth of Scientific Knowledge