 In some texts, Leibniz suggests that the sufficient reason for contingent truths cannot be found in the concepts or natures of things. We must instead look to the Principle of the Best (Mon. 36–38/G VI, 613). In other words the sufficient reason for any contingent proposition of the form “a is F” is that a is F is true in the best possible word. This appears to be an entirely different sort of reason than the fact that “a is F” is analytic or that the nature of a determines that it is F. Those reasons are to be found internal to the concept of the subject or the nature of the substance. Reasons that advert to the Principle of the Best look outside the concept of the subject or the nature of the substance and make comparisons between worlds in terms of their relative perfections. It is possible that Leibniz thought that these different conceptions of sufficient reasons were equivalent but that they are so is far from obvious.