“Do not dwell upon the sins and mistakes of yesterday so exclusively as to have no energy and mind left for living rightly today, and do not think that the sins of yesterday can prevent you from living purely today.”

Source: Into the Dreaming

On Literature, Revolution, Entropy and Other Matters (1923)
Context: A new form is not intelligible to everyone; many find it difficult. Perhaps. The ordinary, the banal is, of course, simpler, more pleasant, more comfortable. Euclid's world is very simple, and Einstein's world is very difficult — but it is no longer possible to return to Euclid. No revolution, no heresy is comfortable or easy. For it is a leap, it is a break in the smooth evolutionary curve, and a break is a wound, a pain. But the wound is necessary: most of mankind suffers from hereditary sleeping sickness, and victims of this sickness (entropy) must not be allowed to sleep, or it will be their final sleep, death.
The same disease often afflicts artists and writers: they sink into satiated slumber in forms once invented and twice perfected. And they lack the strength to wound themselves, to cease loving what they once loved, to leave their old, familiar apartments filled with the scent of laurel leaves and walk away into the open field, to start anew.
Of course, to wound oneself is difficult, even dangerous. But for those who are alive, living today as yesterday and yesterday as today is still more difficult.

Included in Portrait-Life of Lincoln (1910) by Francis T Miller
Posthumous attributions

On Literature, Revolution, Entropy and Other Matters (1923)
Context: A literature that is alive does not live by yesterday's clock, nor by today's but by tomorrow's. It is a sailor sent aloft: from the masthead he can see foundering ships, icebergs, and maelstroms still invisible from the deck. He can be dragged down from the mast and put to tending the boilers or working the capstan, but that will not change anything: the mast will remain, and the next man on the masthead will see what the first has seen.
In a storm, you must have a man aloft. We are in the midst of storm today, and SOS signals come from every side.

Source: Love, Stargirl

היום אתה חש מרומם. אל תתן לימות האתמול והמחר להשפיל את רוחך
Hayom ata hash m'romam. Al titen l'ymot ha'etmol v'hamahar lehashpil at ruhekha.
Attributed