“We must grant the artist his subject, his idea, what the French call his donnée; our criticism is applied only to what he makes of it. Naturally I do not mean that we are bound to like it or find it interesting: in case we do not our course is perfectly simple — to let it alone. We may believe that of a certain idea even the most sincere novelist can make nothing at all, and the event may perfectly justify our belief; but the failure will have been a failure to execute, and it is in the execution that the fatal weakness is recorded. If we pretend to respect the artist at all we must allow him his freedom of choice, in the face, in particular cases, of innumerable presumptions that the choice will not fructify. Art derives a considerable part of its beneficial exercise from flying in the face of presumptions, and some of the most interesting experiments of which it is capable are hidden in the bosom of common things.”

Introduction "On The Sources of Knowledge and of Ignorance" Section XVII, p. 30 Variant translation: I believe it is worthwhile trying to discover more about the world, even if this only teaches us how little we know. It might do us good to remember from time to time that, while differing widely in the various little bits we know, in our infinite ignorance we are all equal.
If we thus admit that there is no authority beyond the reach of criticism to be found within the whole province of our knowledge, however far we may have penetrated into the unknown, then we can retain, without risk of dogmatism, the idea that truth itself is beyond all human authority. Indeed, we are not only able to retain this idea, we must retain it. For without it there can be no objective standards of scientific inquiry, no criticism of our conjectured solutions, no groping for the unknown, and no quest for knowledge.
Conjectures and Refutations: The Growth of Scientific Knowledge (1963)

1860s, Our Composite Nationality (1869)
Context: It is objected to the Chinaman that he is secretive and treacherous, and will not tell the truth when he thinks it for his interest to tell a lie. There may be truth in all this; it sounds very much like the account of man’s heart given in the creeds. If he will not tell the truth, except when it is for his interest to do so, let us make it for his interest to tell the truth. We can do it by applying to him the same principle of justice that we apply to ourselves. But I doubt if the Chinese are more untruthful than other people. At this point I have one certain test. Mankind are not held together by lies. Trust is the foundation of society. Where there is no truth, there can be no trust, and where there is no trust, there can be no society. Where there is society, there is trust, and where there is trust, there is something upon which it is supported. Now a people who have confided in each other for five thousand years; who have extended their empire in all directions until it embraces one-fifth of the population of the globe; who hold important commercial relations with all nations; who are now entering into treaty stipulations with ourselves, and with all the great European powers, cannot be a nation of cheats and liars, but must have some respect for veracity. The very existence of China for so long a period, and her progress in civilization, are proofs of her truthfulness. This is the last objection which should come from those who profess the all-conquering power of the Christian religion. If that religion cannot stand contact with the Chinese, religion or no religion, so much the worse for those who have adopted it. It is the Chinaman, not the Christian, who should be alarmed for his faith. He exposes that faith to great dangers by exposing it to the freer air of America. But shall we send missionaries to the heathen to right to come to us? I think a few honest believers in the teachings of Confucius would be well employed in expounding his doctrines among us.

1962, Rice University speech

The Basic Teachings - Part 1: Principles of the Teaching (2009)

Writings of the Young Marx on Philosophy and Society, L. Easton, trans. (1967), p. 36
Reflections of a Youth on Choosing an Occupation (1835)

Source: Hearing God: Developing a Conversational Relationship with God

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 233.