  Jevons introduces the logical alphabet – a series of combinations that can be formed with a given set of terms. For instance, A and B produce the four combinations AB, Ab, aB and ab. Using the logical alphabet, logic becomes simply an exercise of fully developing all terms and eliminating the contradictory terms. However, when the amount of letters increases, the amount of possible combinations becomes considerable. Jevons considers some techniques and devices to facilitate these endeavours, such as a ‘Logical slate’ (the logical alphabet engraved upon a school writing slate). Nevertheless, when more than six terms are involved, it becomes almost impossible to solve the problem. To facilitate this kind of reasoning Jevons developed a logical abacus, which operates on simple mechanical principles. It can be seen as one of the first computers.