William Stanley Jevons (1 September 1835 – 13 August 1882) was a British economist and logician.

Irving Fisher described his book The Theory of Political Economy (1871) as beginning the mathematical method in economics. It made the case that economics as a science concerned with quantities is necessarily mathematical. In so doing, it expounded upon the "final" (marginal) utility theory of value. Jevons' work, along with similar discoveries made by Carl Menger in Vienna (1871) and by Léon Walras in Switzerland (1874), marked the opening of a new period in the history of economic thought. Jevons' contribution to the marginal revolution in economics in the late 19th century established his reputation as a leading political economist and logician of the time.

Jevons broke off his studies of the natural sciences in London in 1854 to work as an assayer in Sydney, where he acquired an interest in political economy. Returning to the UK in 1859, he published General Mathematical Theory of Political Economy in 1862, outlining the marginal utility theory of value, and A Serious Fall in the Value of Gold in 1863. For Jevons, the utility or value to a consumer of an additional unit of a product is inversely related to the number of units of that product he already owns, at least beyond some critical quantity.

It was for The Coal Question (1865), in which he called attention to the gradual exhaustion of the UK's coal supplies, that he received public recognition. The most important of his works on logic and scientific methods is his Principles of Science (1874), as well as The Theory of Political Economy (1871) and The State in Relation to Labour (1882).

Jevons was born in Liverpool, in the UK. His father, Thomas Jevons, a man of strong scientific tastes and a writer on legal and economic subjects, was an iron merchant. His mother Mary Anne Jevons was the daughter of William Roscoe. At the age of fifteen he was sent to London to attend University College School. He appears at this time to have already formed the belief that important achievements as a thinker were possible to him, and at more than one critical period in his career this belief was the decisive factor in determining his conduct. Towards the end of 1853, after having spent two years at University College, where his favourite subjects were chemistry and botany, he unexpectedly received the offer of the assayership to the new mint in Australia. The idea of leaving the UK was distasteful, but pecuniary considerations had, in consequence of the failure of his father's firm in 1847, become of vital importance, and he accepted the post.

He left the UK for Sydney in June 1854, and remained there for five years. At the end of that period he resigned his appointment, and in the autumn of 1859 entered again as a student at University College London, proceeding in due course to the B.A. and M.A. degrees of the University of London. He now gave his principal attention to the moral sciences, but his interest in natural science was by no means exhausted: throughout his life he continued to write occasional papers on scientific subjects, and his intimate knowledge of the physical sciences greatly contributed to the success of his chief logical work, The Principles of Science. Not long after taking his M.A. degree Jevons obtained a post as tutor at Owens College, Manchester.

In 1866 he was elected professor of logic and mental and moral philosophy and Cobden professor of political economy in Owens college. Next year he married Harriet Ann Taylor, whose father, John Edward Taylor, had been the founder and proprietor of the Manchester Guardian. Jevons suffered a good deal from ill health and sleeplessness, and found the delivery of lectures covering so wide a range of subjects very burdensome. In 1876 he was glad to exchange the Owens professorship for the professorship of political economy in University College, London. Travelling and music were the principal recreations of his life; but his health continued to be bad, and he suffered from depression. He found his professorial duties increasingly irksome, and feeling that the pressure of literary work left him no spare energy, he decided in 1880 to resign the post. On the 13 August 1882 he was drowned whilst bathing near Hastings.

Throughout his life he had pursued with devotion and industry the ideals with which he had set out, and his journal and letters display a noble simplicity of disposition and an unswerving honesty of purpose. He was a prolific writer, and at the time of his death he occupied the foremost position in the UK both as a logician and as an economist. Alfred Marshall said of his work in economics that it "will probably be found to have more constructive force than any, save that of Ricardo, that has been done during the last hundred years." At the time of his death he was engaged upon an economic work that promised to be at least as important as any that he had previously undertaken. It would be difficult to exaggerate the loss which logic and political economy sustained through the accident by which his life was prematurely cut short.

Stanley Jevons was raised a Christian Unitarian, and exercepts from this journals indicate he remained committed to his Christian beliefs until death. After dying in 1882 from accidental drowning, he was buried in the Hampstead Cemetery.

Jevons arrived quite early in his career at the doctrines that constituted his most characteristic and original contributions to economics and logic. The theory of utility, which became the keynote of his general theory of political economy, was practically formulated in a letter written in 1860; and the germ of his logical principles of the substitution of similars may be found in the view which he propounded in another letter written in 1861, that "philosophy would be found to consist solely in pointing out the likeness of things." The theory of utility above referred to, namely, that the degree of utility of a commodity is some continuous mathematical function of the quantity of the commodity available, together with the implied doctrine that economics is essentially a mathematical science, took more definite form in a paper on "A General Mathematical Theory of Political Economy," written for the British Association in 1862. This paper does not appear to have attracted much attention either in 1862 or on its publication four years later in the Journal of the Statistical Society; and it was not till 1871, when the Theory of Political Economy appeared, that Jevons set forth his doctrines in a fully developed form.

It was not till after the publication of this work that Jevons became acquainted with the applications of mathematics to political economy made by earlier writers, notably Antoine Augustin Cournot and HH Gossen. The theory of utility was at about 1870 being independently developed on somewhat similar lines by Carl Menger in Austria and Leon Walras in Switzerland. As regards the discovery of the connection between value in exchange and final (or marginal) utility, the priority belongs to Gossen, but this in no way detracts from the great importance of the service which Jevons rendered to British economics by his fresh discovery of the principle, and by the way in which he ultimately forced it into notice. In his reaction from the prevailing view he sometimes expressed himself without due qualification: the declaration, for instance, made at the commencement of the Theory of Political Economy, that value depends entirely upon utility, lent itself to misinterpretation. But a certain exaggeration of emphasis may be pardoned in a writer seeking to attract the attention of an indifferent public. The Neoclassical Revolution, which would reshape economics, had been started.

Jevons did not explicitly distinguish between the concepts of ordinal and cardinal utility. Cardinal utility allows the relative magnitude of utilities to be discussed, while ordinal utility only implies that goods can be compared and ranked according to which good provided the most utility. Although Jevons predated the debate about ordinality or cardinality of utility, his mathematics required the use of cardinal utility functions. For example, in The Theory of Political Economy, Chapter II, the subsection on "Theory of Dimensions of Economic Quantities," Jevons makes the statement that "In the first place, pleasure and pain must be regarded as measured upon the same scale, and as having, therefore, the same dimensions, being quantities of the same kind, which can be added and subtraced...." Speaking of measurement, addition, and subtraction requires cardinality, as does Jevons' heavy use of integral calculus. Note that cardinality does NOT imply direct measurability, in which Jevons did not believe.

It was not, however, as a theorist dealing with the fundamental data of economic science, but as a brilliant writer on practical economic questions, that Jevons first received general recognition. A Serious Fall in the Value of Gold (1863) and The Coal Question (1865) placed him in the front rank as a writer on applied economics and statistics; and he would be remembered as one of the leading economists of the 19th century even had his Theory of Political Economy never been written. Amongst his economic works may be mentioned Money and the Mechanism of Exchange (1875), written in a popular style, and descriptive rather than theoretical, but wonderfully fresh and original in treatment and full of suggestiveness, a Primer on Political Economy (1878), The State in Relation to Labour (1882), and two works published after his death, namely, Methods of Social Reform and Investigations in Currency and Finance, containing papers that had appeared separately during his lifetime. The last - named volume contains Jevons's speculations on the connection between commercial crises and sunspots. He was engaged at the time of his death upon the preparation of a large treatise on economics and had drawn up a table of contents and completed some chapters and parts of chapters. This fragment was published in 1905 under the title of The Principles of Economics: a fragment of a treatise on the industrial mechanism of society, and other papers.

In The Coal Question, Jevons covered a breadth of concepts on energy depletion that have recently been revisited by writers covering the subject of peak oil. For example, Jevons explained that improving energy efficiency typically reduced energy costs and thereby increased rather than decreased energy use, an effect now known as the Jevons paradox. The Coal Question remains a paradigmatic study of resource depletion theory. Jevons's son, H. Stanley Jevons, published an 800 page follow up study in 1915 in which the difficulties of estimating recoverable reserves of a theoretically finite resource are discussed in detail.

In a relatively minor work, “Commercial Crises and Sun - Spots”, Jevons analyzed business cycles, proposing that crises in the economy might not be random events, but might be based on discernible prior causes. To clarify the concept, he presented a statistical study relating business cycles with sunspots. His reasoning was that sunspots affected the weather, which, in turn, affected crops. Crops changes could then be expected to cause economic changes.

Jevons' work in logic went on pari passu 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 considered the false mathematical dress of that system. In the years immediately following he devoted considerable attention to the construction of a logical machine, exhibited before the Royal Society in 1870, by means of which the conclusion derivable from any given set of premisses could be mechanically 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 applications including the "Logic Piano", a mechanical computer he designed and had built in 1869.

In the following year appeared the Elementary Lessons on Logic, which soon became the most widely read elementary textbook on logic in the English language. In the meantime he was engaged upon a much more important 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 induction is simply an inverse employment of deduction; he treated in a luminous manner the general theory of probability, and the relation between probability 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, often worked out in great detail. An example is his discussion of the use of one - way functions in cryptography, including remarks on the integer factorization problem that foreshadowed its use in public key cryptography. Jevons' general theory of induction was a revival of the theory laid down by Whewell and criticized by John Stuart Mill; but it was put in a new form, and was free from some of the non - essential adjuncts which rendered Whewell's exposition open to attack. The work as a whole was one of the most notable contributions to logical doctrine that appeared in the UK in the 19th century. His Studies in Deductive Logic, consisting mainly of exercises and problems for the use of students, was published in 1880. In 1877 and the following years Jevons contributed to the Contemporary Review some articles on Mill, which he had intended to supplement by further articles, and eventually publish in a volume as a criticism of Mill's philosophy. These articles and one other were republished after Jevons' death, together with his earlier logical treatises, in a volume, entitled Pure Logic, and other Minor Works. The criticisms on Mill contain much that is ingenious and much that is forcible, but on the whole they cannot be regarded as taking rank with Jevons's other work. His strength lay in his power as an original thinker rather than as a critic; and he will be remembered by his constructive work as logician, economist and statistician.

Jevons had written in his Principles of Science, p. 123, "Can the reader say what two numbers multiplied together will produce the number 8616460799 ? I think it unlikely that anyone but myself will ever know." This became known as Jevons' Number and was factored by Derrick Norman Lehmer in 1903 and later on a pocket calculator by Solomon W. Golomb.