Doubt, the Engine of Inquiry

Paul Brunton, Notebooks, vol. 13, part II, p. 10, #48:

It is the first operation of  philosophical training to instill doubt, to free the mind of all those numerous suggestions and distortions imposed on it by others since childhood and maintained by its own slavish acceptance, total unawareness, or natural incapacity.

Or as I have put it more than once in these pages: Doubt is the engine of inquiry, the motor of mental development. Of course, doubting  and questioning are not ends in themselves, but means to the attainment of such insight as it is possible for us to attain.