The term homo economicus—a Linnaeus-themed variant of John Mill's 1836 generic utility maximizing "economic man"—is said to be attributed to the work of Italian engineer and physical socioeconomicist Vilfredo Pareto and his 1896 statement that: "man himself; stripping him of a large number of his attributes, leaving out the passions, good or bad, reducing him to a kind of molecule that only acts in response to the forces of ophelimity”, which, as summarized by Australian economics historian Michael McLure (2002), is an abstract entity that only responds to the forces of ophelimity, in other words "an abstract molecule that acts only in response to economic forces". [1]

“Pareto’s homo economicus is ‘an abstract molecule that responds only to economic forces’.”

— Michael McLure (2002), Pareto, Economics and Society [1]

“Homo economicus knows neither benevolence nor malevolence, only indifference. Human beings in this model are first, last, and always extreme individuals: economic molecules, in the conception of Leon Walras, one of the founders of modern economics.”

“The ruling idea in economic studies following those of Adam Smith was wealth. Later it became the idea of value and is so still in the case of many writers. The science has contained in a sporadic shape much material for a science of economic equilibrium, such as has been suggested by Pareto in a work which makes only a beginning of a study of economic dynamics.

In describing dynamic phenomena comparisons are used which were formerly taken from mechanics, but now more usually from biology. Economic agents are thought of either as molecules subjected to equal pressure in all directions, or, on the other hand, as parts of a living body subjected to equal stimuli, which are mutually counteracting. There is little use in disputing as to which method is better, since the useful thing is to apply a method rather than to argue about it.

Economic dynamics may be defined as a study of movements of disequilibrium, which lead to positions of equilibrium. When an individual spends his income so as to bring into a proportion the marginal utility of different articles within his purchasing power, the equilibrium exists and is rightly called static, because it will continue indefinitely and return if disturbed. Until this condition is reached, modifications in his demand or in his supply are likely to take place, and the quantity of the goods coming within his reach will change and these changes affect both the man himself and the persons he deals with, involving both the quantity of goods available and the incomes of different producers. The state of equilibrium [see: equilibrium state] yields the maximum of satisfaction relatively to the initial position and to the changes which this allows.

In every society purely economic motives have an extending or contracting radius of activity; in other words, the zones of economic action grow larger or smaller. As Pareto says:

“Man’s actual conduct resembles that of the homo economicus, or that of the homo ethicus, or that of the homo religiosus. It is sometimes a composite of all these characters. There are concrete phenomena in which the economic influences transcend all others, and here it is possible to consider alone the results deduced by economic reasoning; while there are other phenomena in which the economic constituent is insignificant and may be neglected. There are still others which are intermediate in character.”

To this we in the main agree, but find it necessary to examine variations of the zones in which all these homines move, and we must notice that even the homo economicus in his own proper capacity is modified when he enters into a composite with the others.”

“In pure theory, homo economicus of Cours [on Political Economy] is analogous to a molecule in the theory of mechanics (note: the impersonal subject pronoun has been used because the homo economicus is an abstract ‘molecule’, not a person). Kirman has pointed out that ‘Pareto regarded equilibrium as the termination point of a process … The time taken for this process is not specified but it certainly is not regarded as … as negligible (Kirman 1987, pg. 806).”

In the 1930s, American physiologist Lawrence Henderson thought, in his Sociology 23 course, that Vilfredo Pareto's 1912 mechanics based socioeconomic equilibrium theory, while not directly based on the chemical thermodynamics based equilibrium theories of American engineer Willard Gibbs, are nevertheless arguing the same point of view, and hence modern socioeconomic equilibrium theories will need to be reformulated in the physical chemistry reaction terms of chemical thermodynamics, a unified synthesis of Gibbs and Pareto, in short. [1]

“Pareto’s work appealed to social scientists who had lost faith in the rationality of the public but not in the rationality of science. Henderson fit the bill perfectly: he was so taken with Pareto’s ideas that he led a faculty seminar on Pareto at Harvard in the early 1930s, from which emerged Henderson’s study Pareto’s General Sociology: a Physiologist’s Interpretation.

Pareto’s mathematical analysis of society was based on his engineer’s understanding of the thermodynamics of equilibrium systems. For example, one of Pareto’s central concepts was that of ‘ophelimity’, now usually called Pareto optimality. Ophelimity was a redefinition of marginal utility in thoroughly thermodynamic terms. Pareto defined a system as having maximum ophelimity when the increase of the ophelimity of any element in the system necessarily reduced that of some other element (or elements) in the system. Thus ophelimity, like energy, always was conserved in a closed system, making the reallocation of ophelimity a zero-sum game. This concept, it should be noted, especially welfare economics, where it plays an important role in the work of Paul Samuelson and Abram Berg.”

“Gibbs considers temperature, pressure, and concentrations, so Pareto considers sentiments, or, strictly speaking, the manifestations of sentiments in words and deeds, verbal elaborations, and …”

“It is very unlikely that the general characteristics of Gibbs’ system had anything to do with Pareto’s construction of his social system. In other words, it is very probable, I thing certain, that Pareto did not keep Gibbs’ work in mind and a fortiori that he did not imitate it, when he worked out his social system; so that Pareto’s system is not the result of the application of the theories of physical chemistry to sociology.”

“This simple example illustrates [the] logical principles [physical chemistry] that find universal application in the physical, biological, and social sciences.”