To 'doubt' in this sense does not imply a psychological state of inability to arrive at decisions or convictions, as is the case in obsessional doubt, but the readiness and capacity for critical questioning of all assumptions and institutions which have become idols under the name of common sense, logic, and what is supposed to be 'natural'.
This radical questioning is possible only if one does not take the concepts of one's own society or even of an entire historical period - like the Western culture since the Renaissance - for granted, and furthermore if one enlarges the scope of one's awareness and penetrates into the unconscious aspects of one's thinking.
Radical doubt is an act of uncovering and discovering; it is the dawning of the awareness that the Emperor is naked, and that his splendid garments are nothing but the products of one's fantasy.
Radical doubt is a process; a process of liberation from idolatrous thinking; a widening of awareness, of imaginative, creative vision of our possibilities and options.
All this means that humanist radicalism questions every idea and every institution from the standpoint of whether it helps or hinders man's capacity for greater aliveness and joy.

[Erich Fromm]
Introduction to 'Celebration of Awareness', by Ivan Illich, p.9-11
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