“Persistence is to the character of man as carbon is to steel.”

Source: Human, All Too Human

On a later meeting of Richard Francis Burton, on 8 February 1879, in, Vol. 1, p. 225
Personal Reminiscences of Henry Irving (1907)
Context: My first impression of the man as of steel was consolidated and enhanced. He told us, amongst other things, of the work he had in hand. Three great books were partially done. The translation of the Arabian Nights, the metrical translation of Camoëns, and the Book of the Sword. These were all works of vast magnitude and requiring endless research. But he lived to complete them all.

Pearls of Wisdom

Response to a student's question in her writing class, as quoted by Louis Menand in the New Yorker (June 8-15, 2009), p. 112.

From 1980s onwards, Cosmography (1992)
Context: The dark ages still reign over all humanity, and the depth and persistence of this domination are only now becoming clear.
This Dark Ages prison has no steel bars, chains, or locks. Instead, it is locked by misorientation and built of misinformation. Caught up in a plethora of conditioned reflexes and driven by the human ego, both warden and prisoner attempt meagerly to compete with God. All are intractably skeptical of what they do not understand.
We are powerfully imprisoned in these Dark Ages simply by the terms in which we have been conditioned to think.

"Interrupting Your Life: An Ethics for the Coming Storm" (2014)

Plan B