Although logical reasoning is necessary for finding the truth among opposing philosophies, it is possible to overestimate its importance. This happens when consistency in a philosophical system is wrongly considered to be its main or even only criterion. A great philosopher will always be prepared to sacrifice a principle which leads to a falsehood and thus to allow his system to become inconsistent. This is illustrated with examples drawn from ethics (Bentham and Mill) and the philosophy of science (Poincaré and Le Roy).