Jevons's work in logic went on poi passe with his work in political economy. In 1864 he published a small volume, entitled Pure Logic; or, the Logic of Quality apart from Quantity, which was based on Boole's system of logic, but freed from what he con sidered the false mathematical dress of that system. In the years immediately following he constructed a logical machine, exhibited before the Royal Society in 1870, by means of which the con clusion derivable from any given set of premisses could be me chanically obtained. In 1866 what he regarded as the great and universal principle of all reasoning dawned upon him ; and in 1869 he published a sketch of this fundamental doctrine under the title of The Substitution of Similars. He expressed the principle in its simplest form as follows : "Whatever is true of a thing is true of its like," and he worked out in detail its various applica tions. In the following year appeared the Elementary Lessons on Logic. In the meantime he was engaged upon a much more im portant logical treatise, which appeared in 1874 under the title of The Principles of Science. In this work Jevons embodied the substance of his earlier works on pure logic and the substitution of similars ; he also enunciated and developed the view that induc tion is simply inverse deduction ; he treated in a luminous manner the general theory of probability, and the relation between proba bility and induction ; and his knowledge of the various natural sciences enabled him throughout to relieve the abstract character of logical doctrine by concrete scientific illustrations. Jevons's
general theory of induction was a revival of the theory laid down by Whewell and criticized by Mill; but it was put in a new form, and was free from some of the non-essential adjuncts which ren dered Whewell's exposition open to attack. The work as a whole was one of the most notable contributions to logical doctrine that appeared in Great Britain in the 19th century. His Studies in Deductive Logic, consisting mainly of exercises and problems for the use of students, was published in 1880. Jevons's strength lay in his power as an original thinker ; and he will be remembered by his constructive work as logician, economist and statistician.
See Letters and Journal of W. Stanley Jevons, edit. by his wife (i886). This work contains a bibliography of Jevons's writings. See also Logic : History.