“By doubting we come to examine, and by examining we reach the truth.”

Introduction as translated in Readings in European History, Vol. I (1904) edited by James Harvey Robinson, p. 451
Variant translation:
Constant and frequent questioning is the first key to wisdom … For through doubting we are led to inquire, and by inquiry we perceive the truth.
Prologue as translated in A History of Education During the Middle Ages and the Transition to Modern Times (1918) by Frank Pierrepont Graves; 2005 edition, p. 53<!-- translation of Prima sapientiae clavis definitur, assidua scilicet seu frequens interrogatio … Dubitando enim ad inquisitionem venimus; inquirendo veritatem percipimus. -->
Sic et Non (1120)
Context: I have ventured to bring together various dicta of the holy fathers, as they came to mind, and to formulate certain questions which were suggested by the seeming contradictions in the statements. These questions ought to serve to excite tender readers to a zealous inquiry into truth and so sharpen their wits. The master key of knowledge is, indeed, a persistent and frequent questioning. Aristotle, the most clear-sighted of all the philosophers, was desirous above all things else to arouse this questioning spirit, for in his Categories he exhorts a student as follows: "It may well be difficult to reach a positive conclusion in these matters unless they be frequently discussed. It is by no means fruitless to be doubtful on particular points." By doubting we come to examine, and by examining we reach the truth.