The 17th-century rationalists adopted the traditional identity of cause and reason, but they inverted the terms: causes were reasons and, most often, reasons were of a mathematical kind. The confusion of cause and reason in favor of the latter was useful for science as long as it rested on an unbounded faith in the rationalizable, intelligible structure of reality, and on confidence in the power of reason to disclose it. One of the most amusing shows provided by rationalism was Christian Wolff’s attempt to demonstrate the principle of sufficient reason—which attempt was, of course, grounded on the assumption of the validity of the very principle he tried to demonstrate. There is a rule, occurring in elementary treatises on probability, which at first sight contradicts the principle of sufficient reason, namely, the so-called principle of indifference, or principle of insufficient reason.