Kuhn on the cost of intellectual pursuits

"Anyone who has wrestled with a project like mine will recognize what it has occasionally cost them."
-Thomas S. Kuhn, "The Structure of Scientific Revolutions," p. xii.

Feynman on Science and Morality

"The sciences do not directly teach good or bad."
from The Value of Science, by Richard P. Feynman

Feynman on Unpolarized Light as a Limitation of Measurement

"...light is unpolarized only if we are unable to find out whether the light is polarized or not."
-Richard Feynman, from "The Feynman Lectures on Physics: Volume 1," p. 33-2.

Socrates on allegiances

"Gentlemen, I am your grateful and devoted servant, but I owe a greater obedience to God than to you; and as long as I draw breath and have my faculties I shall never stop practicing philosophy"
Socrates, from Plato's The Apology

Ernest Rutherford on statistics

"If your experiment needs statistics, you ought to have done a better experiment."
Ernest Rutherford

Socrates on death

"Some one will say: And are you not ashamed, Socrates, of a course of life which is likely to bring you to an untimely end? To him I may fairly answer: There you are mistaken: a man who is good for anything ought not to calculate the chance of living or dying; he ought only to consider whether in doing anything he is doing right or wrong--acting the part of a good man or of a bad..."
Socrates, from Plato's The Apology

I am not alone

It really is quite amazing when you stumble upon a quote from a historical figure that so precisely reflects your state of mind that it gives you a slight chill when you first read it. Such was my reaction upon reading a quote from Kierkegaard on Wikipedia's entry on existentialism:

What I really lack is to be clear in my mind what I am to do, not what I am to know, except in so far as a certain knowledge must precede every action. The thing is to understand myself, to see what God really wishes me to do: the thing is to find a truth which is true for me, to find the idea for which I can live and die. ... I certainly do not deny that I still recognize an imperative of knowledge and that through it one can work upon men, but it must be taken up into my life, and that is what I now recognize as the most important thing.

—Søren Kierkegaard, Letter to Peter Wilhelm Lund dated August 31, 1835, emphasis added