No man would engage in the pursuit of philosophy if he thought that all philosophy is merely an expression of irrational bias.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“Subjectively, every philosopher appears to himself to be engaged in the pursuit of something which may be called “truth.” Philosophers may differ as to the definition of “truth,” but at any rate it is something objective, something which, in some sense, everybody ought to accept. No man would engage in the pursuit of philosophy if he thought that all philosophy is merely an expression of irrational bias. But every philosopher will agree that many other philosophers have been actuated by bias, and have had extra-rational reasons, of which they were usually unconscious, for many of their opinions. Marx, like the rest, believes in the truth of his own doctrines;” source