“[I]s this not an advantage? Is it not a sign of immense progress that the masses should have "ideas," that is to say, should be cultured? By no means. The "ideas" of the average man are not genuine ideas, nor is their possession culture. An idea is a putting truth in checkmate. Whoever wishes to have ideas must first prepare himself to desire truth and to accept the rules of the game imposed by it. It is no use speaking of ideas when there is no acceptance of a higher authority to regulate them, a series of standards to which it is possible to appeal in a discussion. These standards are the principles on which culture rests.”

"On Freedom" in All Life is Problem Solving (1999)
Context: When I speak of reason or rationalism, all I mean is the conviction that we can learn through criticism of our mistakes and errors, especially through criticism by others, and eventually also through self-criticism. A rationalist is simply someone for whom it is more important to learn than to be proved right; someone who is willing to learn from others — not by simply taking over another's opinions, but by gladly allowing others to criticize his ideas and by gladly criticizing the ideas of others. The emphasis here is on the idea of criticism or, to be more precise, critical discussion. The genuine rationalist does not think that he or anyone else is in possession of the truth; nor does he think that mere criticism as such helps us achieve new ideas. But he does think that, in the sphere of ideas, only critical discussion can help us sort the wheat from the chaff. He is well aware that acceptance or rejection of an idea is never a purely rational matter; but he thinks that only critical discussion can give us the maturity to see an idea from more and more sides and to make a correct judgement of it.

Letter to Hugo Boxel (Oct. 1674) The Chief Works of Benedict de Spinoza (1891) Tr. R. H. M. Elwes, Vol. 2, Letter 58 (54).
Context: If I had as clear an idea of ghosts, as I have of a triangle or a circle, I should not in the least hesitate to affirm that they had been created by God; but as the idea I possess of them is just like the ideas, which my imagination forms of harpies, gryphons, hydras, &c., I cannot consider them as anything but dreams, which differ from God as totally as that which is not differs from that which is.<!--pp. 382-383

Source: The Book of Nothing (2009), chapter nought "Nothingology—Flying to Nowhere"

Source: Society of the Spectacle (1967), Ch. 8, sct. 207 (confer Comte de Lautréamont, Poésies II, 1870).

Source: The Emergence Of Probability, 1975, Chapter 15, Inductive Logic, p. 139.

A Writer's Diary, Volume 1: 1873-1876 (1994), p. 734 http://books.google.com.br/books?id=38xQHS4h0yEC&printsec=frontcover&hl=pt-BR&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

(original Dutch, citaat van B.C. Koekkoek:) De vrije kunstenaar moet ook den moed bezitten zich van zijn eersten ideën te kunnen losrukken, want de ondervinding leert ons, dat dezelve niet altijd zuiver, ja dikwijls valsch zijn.
Source: Herinneringen aan en Mededeelingen van…' (1841), p. 101