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in researches and speculations connected with the science of logic. He had become discontented with Mill, and resented Mill's indifference to Boole's speculations. In his ‘Pure Logic’ (1864) he had already put forward a system based on the conclusions of Boole, and in the following year he completed the construction of his ‘reasoning machine, or logical abacus, adapted to show the working of Boole's logic in a half mechanical manner,’ which in March and April 1866 he exhibited to the Liverpool and Manchester Literary and Philosophical Societies (described in his paper ‘On the Mechanical Contrivance of Logical Inference,’ read before the Royal Society in January 1870, and printed in the ‘Philosophical Transactions,’ clx. 497 sqq.) When staying in the Isle of Wight with his wife in the autumn of 1868 he read to her three articles directed against Mill's logical system, which were, however, refused by a leading magazine (Letters and Journal, pp. 244–5). Their spirit may have been condensed into certain trenchant passages of his little treatise on ‘The Substitution of Similars,’ which he published in the following year (1869), with a frontispiece representing the logical abacus, and which, while conveying his theory of reasoning in outline, was designed as an uncompromising step towards the liberation of logic from the ban of metaphysics, and its establishment as an exact science. He returned to the subject in a paper read in January 1870 before the Manchester Literary and Philosophical Society ‘On a General System of Numerically Definite Reasoning;’ and in his ‘Elementary Lessons in Logic’ (also 1870) he sought to give a clear notion of the results to which the ‘discoveries’ of Boole and his predecessors necessarily led. The ‘Studies in Deductive Logic,’ which followed several years later (1880), consist of a series of logical problems intended to carry on and exemplify the chief purpose of the ‘Elementary Lessons.’ Jevons's ‘Primer of Logic’ was published in 1876.

The most important, however, of this group of his works was his ‘Principles of Science,’ 2 vols., 1874; 2nd edit., 1 vol., 1877. In this book, with illustrations derived from almost every branch of scientific research, he developed his theory of logic and scientific method, and of its applicability beyond and, so to speak, above the sphere of physical science alone. This work, which proved more stimulative to mathematicians than to metaphysicians (see the Pref. to 2nd edit.), was to have been followed by an ‘Analysis of Mill's “Analysis of Knowledge,”’ of which the substance is probably to be found in a series of papers in the ‘Contemporary Review’ (December 1877, January and April 1878). These papers are reprinted, together with some of the author's earliest contributions to the science of logic, in ‘Pure Logic and other Minor Works. By W. S. Jevons. Edited by R. Adamson and H. A. Jevons, 1890.’ In the preface Professor Adamson examines the essential difference between Mill's and Jevons's points of view in logical theory, and with the aid of Jevons's unpublished manuscripts exhibits the general plan of attack which he proposed to himself, but was only partially able to carry out.

Jevons was a frequent attendant at the meetings of the Manchester and London Statistical societies; to the latter he had been elected in 1864. On 17 Nov. 1868 he read there a paper ‘On the Condition of the Metallic Currency of the United Kingdom,’ in which he urged a recoinage and the introduction of an international money. In order to estimate the age of the gold circulation in England, and the loss on light gold coins, he had previously, by means of returns with which he was furnished by bankers, made a census of over 165,000 gold sovereigns and half-sovereigns. He followed up the subject by two letters to the ‘Times’ (27 Aug. and 7 Sept. 1869). The paper, with other cognate studies belonging to this period, is reprinted in the posthumous ‘Investigations in Currency and Finance,’ 1884, which also include a previously unpublished paper written in 1875, and entitled ‘An Ideally Perfect System of Currency.’ In April 1870 he delivered a lecture on ‘Industrial Partnerships,’ ‘under the auspices of the Social Science Association.’ In March 1869 he had been consulted by Mr. Lowe, the chancellor of the exchequer, upon questions of taxation; and his advice for abolishing the duty of a shilling a quarter upon corn had been actually followed in the budget. In 1871 his masterly pamphlet on the ‘Match Tax: a Problem in Finance,’ vindicated the policy of the same financier, though after the battle had been lost.

In the same year Jevons opened a fresh view of research in his ‘Theory of Political Economy.’ Regarding political economy as a mathematical science, ‘in matter if not in language,’ he attempted to put its main definitions in the shape of quantitative formulæ, and in the process, though not highly accomplished as a mathematician, or altogether at his ease when using mathematical language, he threw much light upon the nature and the mutual relations of economic quantities themselves (cf. Professor A. Marshall ap. Harley, p. ix). In his paper ‘On the