“If a man has not, by the time he is 30, yielded to the fascination of every form of extremism, I don't know if he is to be admired or scorned — a saint or a corpse.”

Source: The Serpent's Tale

Oxford Union Debate (3 December 1964)

Misattributed

Martin Luther, quoted at the beginning of The Screwtape Letters
Misattributed

Author’s Warning
Source: Memory, Sorrow, and Thorn, The Dragonbone Chair (1988)

As quoted in Many Thoughts of Many Minds (1862) edited by Henry Southgate, p. 290

Hyperion http://www.gutenberg.org/etext/5436, Bk. III, Ch. IV (1839).
Variant: Believe me, every heart has its secret sorrows, which the world knows not, and oftentimes we call a man cold, when he is only sad.
Context: "Ah! this beautiful world!" said Flemming, with a smile. "Indeed, I know not what to think of it. Sometimes it is all gladness and sunshine, and Heaven itself lies not far off. And then it changes suddenly; and is dark and sorrowful, and clouds shut out the sky. In the lives of the saddest of us, there are bright days like this, when we feel as if we could take the great world in our arms and kiss it. Then come the gloomy hours, when the fire will neither burn on our hearths nor in our hearts; and all without and within is dismal, cold, and dark. Believe me, every heart has its secret sorrows, which the world knows not, and oftentimes we call a man cold, when he is only sad."

Heath's book of Beauty, 1833 (1832)

but I do not envy them. For if anything is capable of making a poet out of a man of letters, it is this plebeian love of mine for the human, living, and commonplace. All warmth, all goodness, all humor is born of it, and it almost seems to me as if it were that love itself, of which it is written that a man might speak with the tongues of men and of angels, and yet without it be no more than sounding brass or a tinkling cymbal.
Source: Tonio Kröger (1903), Ch. 9, as translated by Bayard Quincy Morgan