“I've come to believe errors, especially written errors, are often the only markers left by a solitary life: to sacrifice them is to lose the angles of personality, the riddle of a soul. In this case a very old soul. A very old riddle.”

On Being a Real Person (1943)
Context: Every human life involves an unfathomable mystery, for man is the riddle of the universe, and the riddle of man is his endowment with personal capacities. The stars are not so strange as the mind that studies them, analyzes their light, and measures their distances.

Aids to Reflection (1873), Aphorism 1

The Divinity College Address (1838)

"Do We Live Again?" an interview with Edison, as quoted in Mr. Edison's New Argument from Design" in The Illustrated London News (3 May 1924).
1920s

Source: Conjectures and Refutations: The Growth of Scientific Knowledge (1963), Ch. 1 "Science : Conjectures and Refutations"
Context: The history of science, like the history of all human ideas, is a history of irresponsible dreams, of obstinacy, and of error. But science is one of the very few human activities — perhaps the only one — in which errors are systematically criticized and fairly often, in time, corrected. This is why we can say that, in science, we often learn from our mistakes, and why we can speak clearly and sensibly about making progress there.

Misattributed to Plato in Laws by Conservapedia http://www.conservapedia.com/Atheism_Quotes. Actual source: William Fleming, as quoted in Prose Quotations from Socrates to Macaulay by Samuel Austin Allibone, 1816–1889. http://www.bartleby.com/349/authors/74.html
Misattributed

Source: Wicked: The Life and Times of the Wicked Witch of the West

Pt. I, The Unknowable; Ch. I, Religion and Science; quoting from "There is some soul of goodness in things evil / Would men observingly distil it out", William Shakespeare, Henry V, act iv. sc. i
First Principles (1862)