Indirect inference or indirect deduction consists of pointing out “what a thing is, by showing that it cannot be anything else”. According to Jevons this is an important method, since “nearly half our logical conclusions rest upon its employment”. The simplest form of indirect inference starts from A = AB. The law of duality implies that b = Ab ·|· ab, or by substitution b = ABb ·|· ab. Since ABb = 0 (a contradiction), it follows that b = ab. Hence, if a metal is an element, it follows that a n