A depiction of knowledge; “The Bookworm” (1850) by Carl Spitzweg.

“It seems that the author’s continued work in the physical sciences caused him to arrive at this strange title. He might have noticed that in the natural sciences very often ethical parables, far removed from the circle of human knowledge, are employed in order to bring about a closer match of the two—and in this sense, in the case of morality, he likely sought to drive the nature the chemical parable back to its mental origins—being that there is, after all, only one nature—and also since, within serene realm of rational freedom, the cloudy tracks of passionate necessity move inexorably through their course, only to be wiped out by a higher hand, and perhaps not completely wiped out in this life.”

— Johann Goethe (1809), “Advertisement” for Elective Affinities, Sep 4

“When you can measure what you are speaking about and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

— William Thomson (1883) [2]

“All knowledge can be divided into two classes: subjects that study the interrelations of two or more persons (history, literature, economics, sociology, law, politics, theology, education, etc.) and those that do not (logic, mathematics, physics, biology, and other natural sciences, grammar, harmony, etc.); [the former are slower] in setting in motion activity that becomes the most important and influential in the world.”

— Lawrence Henderson (1935), on Machiavelli's studies vs. Galileo's studies [3]

“Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and unbearable pity for the suffering of mankind.”

— Bertrand Russell (1956), “What I Have Lived For”, Prologue to Autobiography [5]

“If devotion to truth is the hallmark of morality, then there is no greater, nobler, more heroic form of devotion than the act of a man who assumes the responsibility of thinking .... the alleged short-cut to knowledge, which is faith, is only a short-circuit destroying the mind.”

— Ayn Rand (1957), Atlas Shrugged (Ѻ)

“No modern chemist, if he is vitally concerned with this experiment as Beard is in discovering the causes of war, would be content with the empirical knowledge that the eleventh chemical somehow did the trick, either by itself, or in activating others. He would want to know just why and how the precipitation came about. He would want to know whether he must always use all of the eleven chemicals the way that he did, or if simpler combinations could effect the desired result. In history, Beard contends, we have an analogous situation. There are ten know factors. An eleventh, let us say, the German unrestricted submarine warfare is added. War is declared. Was that the cause of war? No one knows. Furthermore it is impossible to know.”

— Morris Zucker (1945), commentary on Charles Beard’s 1936 chemical analogy argument against causation in history [4]