“The reason why there are hardly ever completely knock-down arguments, except between very like minded philosophers, is that philosophers, unlike chemists or geologists, are licensed to question everything, including methodology.”

Source: 1930s-1951, Philosophical Occasions 1912-1951 (1993), Ch. 9 : Philosophy, p. 165
Corresponding to TS 213, Kapitel 87, 409

Source: Commentary on the Metaphysics (c. 1270–1272), 1, 3; quoted in Josef Pieper, Leisure, the Basis of Culture (New York, 1952), p. 88

Article on Philosophy, Vol. 25, p. 667, as quoted in Main Currents of Western Thought : Readings in Western European Intellectual History from the Middle Ages to the Present (1978) by Franklin Le Van Baumer
Variant translation: Reason is to the philosopher what grace is to the Christian. Grace moves the Christian to act, reason moves the philosopher. Other men walk in darkness; the philosopher, who has the same passions, acts only after reflection; he walks through the night, but it is preceded by a torch. The philosopher forms his principles on an infinity of particular observations. … He does not confuse truth with plausibility; he takes for truth what is true, for forgery what is false, for doubtful what is doubtful, and probable what is probable. … The philosophical spirit is thus a spirit of observation and accuracy.
L'Encyclopédie (1751-1766)
Context: Reason is to the philosopher what grace is to the Christian.
Grace causes the Christian to act, reason the philosopher. Other men are carried away by their passions, their actions not being preceded by reflection: these are the men who walk in darkness. On the other hand, the philosopher, even in his passions, acts only after reflection; he walks in the dark, but by a torch.
The philosopher forms his principles from an infinity of particular observations. Most people adopt principles without thinking of the observations that have produced them, they believe the maxims exist, so to speak, by themselves. But the philosopher takes maxims from their source; he examines their origin; he knows their proper value, and he makes use of them only in so far as they suit him.
Truth is not for the philosopher a mistress who corrupts his imagination and whom he believes to be found everywhere; he contents himself with being able to unravel it where he can perceive it. He does not confound it with probability; he takes for true what is true, for false what is false, for doubtful what is doubtful, and probable what is only probable. He does more, and here you have a great perfection of the philosopher: when he has no reason by which to judge, he knows how to live in suspension of judgment...
The philosophical spirit is, then, a spirit of observation and exactness, which relates everything to true principles...

Last lines. Hawking later wrote: "In the proof stage I nearly cut the last sentence in the book... Had I done so, the sales might have been halved.
Black Holes and Baby Universes and Other Essays (1993)

Gilles Deleuze & Félix Guattari, What is Philosophy? (cited in the Internet Encyclopedia of Philosophy http://www.iep.utm.edu/d/deleuze.htm#SH3b)
A - F, Gilles Deleuze

Source: The Complex Vision (1920), Chapter I
Context: My answer to the question "Why do we philosophize?" is as follows. We philosophize for the same reason that we move and speak and laugh and eat and love. In other words, we philosophize because man is a philosophical animal.… We may be as sceptical as we please. Our very scepticism is the confession of an implicit philosophy.

Source: Leisure, the Basis of Culture (1948), The Philosophical Act, P. 63

Source: Man Against Mass Society (1952), p. 116