Notes from Wikiquote
As quoted in The Anchor Book of French Quotations with English Translations (1963) by Norbert Gutermam
Variants
A thing is not proved just because no one has ever questioned it. What has never been gone into impartially has never been properly gone into. Hence skepticism is the first step toward truth. It must be applied generally, because it is the touchstone. || The first step towards philosophy is incredulity.