The difficulties of economics are mainly the difficulties of conceiving clearly and fully the conditions of utility. - William Stanley Jevons

By a commodity we shall understand any object, substance, action or service, which can afford pleasure or ward off pain.

Among minor alterations, I may mention the substitution for the name political economy of the single convenient term economics. I cannot help thinking that it would be well to discard, as quickly as possible, the old troublesome double-worded name of our science.

Pray accept my best thanks for your kindness in sending me a copy of your Memoir, and for the very courteous letter in which you draw my attention to it. When your letter came I had, indeed, already noticed in the Journal des Economistes your very remarkable theory. I felt the greater interest in the subject because my own speculations have led me in the same direction, now for the last twelve years or more. It is satisfactory to me to find that my theory of exchange, which, when published in England, was either neglected or criticised, is practically confirmed by your researches. I do not know whether you are acquainted with my writings on the subject. All the chief points of my mathematical theory were clear to my own mind by the year 1862, when I drew up a brief account of it, which was read at the meeting of the British Association at Cambridge... A very brief abstract was then alone inserted in the report, but the original paper was printed in the journal of the London Statistical Society in 1866... Finally, in 1871, I caused to be published... the Theory of Political Economy, in which is given a full explanation of the theory, with the aid of mathematical symbols. [...] You will find, I think, that your theory substantially coincides with and confirms mine, although the symbols are differently chosen, and there are incidental variations. You will see that the whole theory rests on the notion... that the utility of a commodity is not proportional to its quantity; what you call the rarity of a commodity appears to be exactly what I called the coefficient of utility at first, and afterwards the degree of utility, which... was really the differential coefficient of the utility considered as the function of the quantity of commodity. The theory of exchange is given... and may be considered to be contained in one sentence. An equation may thus be established on either side between the utility gained and sacrificed at the ratio of exchange of the whole commodities, upon the last increments exchanged.
Now in my book of 1871, I show fully how this theory may be expressed in symbols. [...] Indeed, when the meaning of the terms is explained, your proposition 'Les prix courants ou prix d'equilibre sont égaux aux rapports des raretés' is seen to coincide precisely with my theory, except that you do not point out how many equations are requisite, or how many unknown quantities there are.
The publication of your paper... tends to confirm my belief in the correctness of the theory, but it might lead to misapprehensions as to the originality and priority... I shall therefore take it as a favour if you will kindly inform me whether you are sufficiently acquainted with my writings, or whether you would desire me to forward a copy of my Theory of Political Economy.