They are men and women who tend to believe that the human being is perfectible and social progress predictable, and that the instrument for effecting the two is reason; that truths are transitory and empirically determined; that equality is desirable and attainable through the action of state power; that social and individual differences, if they are not rational, are objectionable, and should be scientifically eliminated; that all people and societies strive to organize themselves upon a rationalist and scientific paradigm.
A definition of what he meant when referring to "liberals"in Up from Liberalism (1959); as quoted in "An American original: appreciating Bill Buckley" by George Shadroui (2003).