“You’re sincere enough.”

Letter to his brother, A.P. Chekhov (October 13, 1888)
Letters

Source: Academy Series - Priscilla "Hutch" Hutchins, Odyssey (2006), Chapter 3 (p. 24)

The Great Infidels (1881)
Context: All the martyrs in the history of the world are not sufficient to establish the correctness of an opinion. Martyrdom, as a rule, establishes the sincerity of the martyr, — never the correctness of his thought. Things are true or false in themselves. Truth cannot be affected by opinions; it cannot be changed, established, or affected by martyrdom. An error cannot be believed sincerely enough to make it a truth.

(zh-TW) 日月催人老，今生有幾何？
真言誠可貴，感語不嫌多。

"Occasional thoughts" (偶感)

Source: Deng Feng-Zhou, "Deng Feng-Zhou Classical Chinese Poetry Anthology". Volume 6, Tainan, 2018: 88.

Letter to his brother Rev. William N. Cleveland (7 November 1882); published in The Writings and Speeches of Grover Cleveland (1892), p. 534.
Context: I feel as if it were time for me to write to someone who will believe what I write.
I have been for some time in the atmosphere of certain success, so that I have been sure that I should assume the duties of the high office for which I have been named. I have tried hard, in the light of this fact, to appreciate properly the responsibilities that will rest upon me, and they are much, too much underestimated. But the thought that has troubled me is, can I well perform my duties, and in such a manner as to do some good to the people of the State? I know there is room for it, and I know that I am honest and sincere in my desire to do well; but the question is whether I know enough to accomplish what I desire.
The social life which seems to await me has also been a subject of much anxious thought. I have a notion that I can regulate that very much as I desire; and, if I can, I shall spend very little time in the purely ornamental part of the office. In point of fact, I will tell you, first of all others, the policy I intend to adopt, and that is, to make the matter a business engagement between the people of the State and myself, in which the obligation on my side is to perform the duties assigned me with an eye single to the interest of my employers. I shall have no idea of re-election, or any higher political preferment in my head, but be very thankful and happy I can serve one term as the people's Governor.

Grassé, Pierre Paul (1977); Evolution of living organisms: evidence for a new theory of transformation. Academic Press, p. 7
Evolution of living organisms: evidence for a new theory of transformation (1977)
Context: For those who would still doubt the relevance of our criticisms, another quote from a paper written by two American biologists, King and Jukes (1969), is presented:. These biologists are convinced that the Darwinian credo is correct, and they accept it. They are quite sincere but they are not critical enough. Biochemists and biologists who adhere blindly to the Darwinism theory search for results that will be in agreement with their theories and consequently orient their research in a given direction, whether it be in the field of ecology, ethology, sociology, demography (dynamics of populations), genetics (so-called evolutionary genetics), or paleontology. This intrusion of theories has unfortunate results: it deprives observations and experiments of their objectivity, makes them biased, and, moreover, creates false problems. Heedful of genetics and demography, Darwinians have seldom taken fossils* into consideration, or, and this is more serious, they have applied the laws of genetics to them without making a critical analysis; considering our ignorance of the relationships between fossils, which in most cases are found very far apart and in distinct beds, this approach can only be arbitrary.

1970's, Conversations with Samuel Beckett and Bram van Velde (1970 - 1972)

The Analects, The Doctrine of the Mean
Context: Sincerity is the way of Heaven. The attainment of sincerity is the way of men. He who possesses sincerity is he who, without an effort, hits what is right, and apprehends, without the exercise of thought — he is the sage who naturally and easily embodies the right way. He who attains to sincerity is he who chooses what is good, and firmly holds it fast. To this attainment there are requisite the extensive study of what is good, accurate inquiry about it, careful reflection on it, the clear discrimination of it, and the earnest practice of it.