“The Herr Doctor does not know about peoples.”

Source: The Adventures of Sherlock Holmes

Source: Rules for Radicals: A Practical Primer for Realistic Radicals (1971), p. 81

The references are to Matthew 10:29 (or Luke 12:6) and Jerusalem Talmud, Sanhedrin 4:8 (37a).
Source: Schindler's Ark (1982), Ch. 2.

On Truth and Lie in an Extra-Moral Sense (1873)
Context: What does man actually know about himself? Is he, indeed, ever able to perceive himself completely, as if laid out in a lighted display case? Does nature not conceal most things from him — even concerning his own body — in order to confine and lock him within a proud, deceptive consciousness, aloof from the coils of the bowels, the rapid flow of the blood stream, and the intricate quivering of the fibers! She threw away the key.

316.
Aes Triplex (1878)
Variant: Even if the doctor does not give a year, even if he hesitates about a month, make one brave push and see what can be accomplished in a week.

Source: Pale Blue Dot: A Vision of the Human Future in Space (1994), p. 159
Context: It is sometimes said that scientists are unromantic, that their passion to figure out robs the world of beauty and mystery. But is it not stirring to understand how the world actually works — that white light is made of colors, that color is the way we perceive the wavelengths of light, that transparent air reflects light, that in so doing it discriminates among the waves, and that the sky is blue for the same reason that the sunset is red? It does no harm to the romance of the sunset to know a little bit about it.

volume I; lecture 3, "The Relation of Physics to Other Sciences"; section 3-6, "Psychology"; p. 3-8
The Feynman Lectures on Physics (1964)