“In a world of false appearances, be rare: transparent, honest, true and sincere.”

Original: (it) Le persone trasparenti si distinguono sempre: sono terribilmente sincere.
Source: prevale.net

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

Original: (it) Essere sinceri è difficile. La sincerità ha i suoi costi. Per questo, essere sinceri, fa la differenza.
Source: prevale.net

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.

Original Stories from Real Life; with Conversations Calculated to Regulate the Affections, and Form the Mind to Truth and Goodness (1788; 1791)
Context: Good habits, imperceptibly fixed, are far preferable to the precepts of reason; but, as this task requires more judgment than generally falls to the lot of parents, substitutes must be sought for, and medicines given, when regimen would have answered the purpose much better. I believe those who examine their own minds, will readily agree with me, that reason, with difficulty, conquers settled habits, even when it is arrived at some degree of maturity: why then do we suffer children to be bound with fetters, which their half-formed faculties cannot break.
In writing the following work, I aim at perspicuity and simplicity of style; and try to avoid those unmeaning compliments, which slip from the tongue, but have not the least connexion with the affections that should warm the heart, and animate the conduct. By this false politeness, sincerity is sacrificed, and truth violated; and thus artificial manners are necessarily taught. For true politeness is a polish, not a varnish; and should rather be acquired by observation than admonition.

[Saussy, Haun, Lee, Pauline, Handler-Spitz, Rivi, A Book to Burn and a Book to Keep (Hidden): Selected Writings, 2016, Columbia University Press, 0231541538, https://books.google.com/books?id=4Xm0CwAAQBAJ, Prefaces, 4]

Original: (it) In questo mondo che cela ingiustizia, egoismo, odio, falsità ed apparenza; tu, imponiti di essere una brava persona.
Source: prevale.net

Introduction: an evolutionary riddle, p. 5
In Gods We Trust: The Evolutionary Landscape of Religion (2002)