 One can immediately see why the premises of a probable deduction must be compatible: the probability of M(i) relative to A(i) is defined only if the denominator of the fraction is not zero, which means that the premises A(i) are compatible. On the other hand, the number of ideas that make both A(i) and M(i) true cannot be greater than the number of ideas that make true M(i); as a consequence, the conditional probability of M(i) cannot be greater than 1. It is equal to 1 when the number of ideas that make true both A(i) and M(i) is equal to the number of ideas that make true A(i) alone, which means that all substitutions of ideas that make true A(i) also make true M(i), i.e., if M(i) is deducible from A(i). In other words, if M(i) is deducible from A(i), its probability relative to A(i) is equal to 1, which means that the probability equals certainty. The probability is zero if no ideas make both A(i) and M(i) true, i.e., if A(i) and M(i) are incompatible. Incompatibility and certainty are thus two extreme cases of probability with values of 0 and 1.