  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 non-element is a non-metal. Jevons refers to this conclusion as the “contrapositive proposition” of the original. The contrapositive of a simple identity A = B is a = ab, and since A = B implies B = A it also follows that b = ab. The two contrapositives taken together let us conclude that a = b. The method of indirect inference can be used to describe a class of objects or a term, given certain conditions. The class is first of all ‘developed’ using the law of duality, then alternative expressions taken from the premises are substituted, and finally all contradictory alternatives are scrapped. The remaining terms may be equated to the term in question.