“Although textbooks (and professors) like to make definite statements indicating that they know what they are talking about, there is in fact a great deal of uncertainty and ambiguity in the world. …we will not evade this question but rather explore (overexplore?) it. …great progress is often made when what was long believed to be true is now seen to be perhaps not the whole truth. Thus the text often uses words… to cause you to think about the uncertainess and even the arbitrariness of much of our current conventions and definitions, to ponder about your acceptance of them.”

Variant: and it ought to make us feel ashamed when we talk like we know what we're talking about when we talk about love.
Source: What We Talk About When We Talk About Love

Great Books: The Foundation of a Liberal Education (1954)

Foundations of the Republic; Speeches and Addresses (1926), p. 451.
1920s

from lecture "What is and What Should be the Role of Scientific Culture in Modern Society", given at the Galileo Symposium in Italy (1964)
The Pleasure of Finding Things Out (1999)

Source: Winnie-the-Pooh

Source: The Intentional Stance (1987), p. 350

Address to the African Methodist Episcopal Zion Church, Boston, Massachusetts (30 July 1903), printed in "Account of the Boston Riot," Boston Globe (31 July 1903) http://web.archive.org/20071031084056/www.historycooperative.org/btw/Vol.7/html/235.html

Source: The Causes of Evolution (1932), Ch. V What is Fitness?, pp. 158-159.
Context: I have given my reasons for thinking that we can probably explain evolution in terms of the capacity for variation of individual organisms, and the selection exercised on them by their environment....
The most obvious alternative to this view is to hold that evolution has throughout been guided by divine power. There are two objections to this hypothesis. Most lines of descent end in extinction, and commonly the end is reached by a number of different lines evolving in parallel. This does not suggest the work of an intelligent designer, still less of an all mighty one. But the moral objection is perhaps more serious. A very large number of originally free-living Crustacea, worms, and so on, have evolved into parasites. In doing so they have lost, to a greater or less extent, their legs, eyes, and brains, and have become in many cases the course of considerable and prolonged pain to other animals and to man. If we are going to take an ethical point of view at all (and we must do so when discussing theological questions), we are, I think, bound to place this loss of faculties coupled with increased infliction of suffering in the same class as moral breakdown in a human being, which can often be traced to genetical causes. To put the matter in a more concrete way, Blake expressed some doubt as to whether God had made the tiger. But the tiger is in many ways an admirable animal. We have now to ask whether God made the tapeworm. And it is questionable whether an affirmative answer fits in either with what we know about the process of evolution or what many of us believe about the moral perfection of God.