“Though what we accept be true, it is a prejudice unless we ourselves have considered and understood why and how it is true.”

Source: Go Ask Alice

The Atheist's Guide to Reality (2011)
Context: Scientism starts with the idea that the physical facts fix all the facts, including the biological ones. These in turn have to fix the human facts—the facts about us, our psychology, and our morality. After all, we are biological creatures, the result of a biological process that Darwin discovered but that the physical facts ordained. As we have just seen, the biological facts can't guarantee that our core morality (or any other one, for that matter) is the right, true, or correct one. If the biological facts can't do it, then nothing can. No moral code is right, correct, true. That's nihilism. And we have to accept it.

Context: The truth may be puzzling. It may take some work to grapple with. It may be counterintuitive. It may contradict deeply held prejudices. It may not be consonant with what we desperately want to be true. But our preferences do not determine what's true. We have a method, and that method helps us to reach not absolute truth, only asymptotic approaches to the truth — never there, just closer and closer, always finding vast new oceans of undiscovered possibilities. Cleverly designed experiments are the key.

Source: The Da Vinci Code

reprinted in 'Zero', ed. Otto Piene and Heinz Mack, Cambridge, Mass; MIT Press 1973, p. 120
Quotes, 1960's, untitled statements in 'Zero 3', (1961)

A Declaration of Independence (12 March 1964) http://teachingamericanhistory.org/library/index.asp?document=1148
Variant: We cannot think of uniting with others, until after we have first united among ourselves. We cannot think of being acceptable to others until we have first proven acceptable to ourselves.
Context: There can be no black-white unity until there is first some black unity. There can be no workers' solidarity until there is first some racial solidarity. We cannot think of uniting with others, until after we have first united among ourselves. We cannot think of being acceptable to others until we have first proven acceptable to ourselves. One can't unite bananas with scattered leaves.

On Being, The Wisdom of Tenderness (transcript) http://www.onbeing.org/program/wisdom-tenderness/transcript/1369 Interview with Krista Tippett, December 24, 2009
From interviews and talks

Introduction: an evolutionary riddle, p. 5
In Gods We Trust: The Evolutionary Landscape of Religion (2002)

"I'm Too Big But I Love To Play" in Ten Thousand Light-Years From Home (1973)