|An artistic rendition of the generic "rational choice" Adam Smith type of "economic agent" acting so as to maximizing utility, or some variation along these lines. |
In 1941, American economist Paul Samuelson, second generation student of Willard Gibbs, published his PhD thesis, in which, according to American financial engineer Andrew Lo, he modeled each person as an "economic agent", and assumed that each individual acted so as to maximize a quantity called ‘expected utility’, a model with which he assumed should be able to predict their behavior in much the same way that physicists predict the behavior of physical objects.
In 1992, American philosopher Alan Nelson published his “Human Molecules” chapter wherein he posited that "economic agents" should be considered as "human molecules", an analogy comparison according to which just as thermodynamics can be reduced in approximation to statistical mechanics, by making certain assumptions about molecular agents (gas molecules), so to ideally should macroeconomics (general equilibrium theory) be reducible to microeconomics (individual choice), by making certain assumptions about economic agents (human molecules). 
In 1999, in promotion of his upcoming book Investigations (2000), American bio-chemist Stuart Kauffman began to outline what seems to be his unified theory, that of what he calls the “adjacent possible” (or chemically adjacent possible), a rather ill-defined term, which he describes as those regions of the biosphere in which “autonomous agents”—Kauffman’s term for a minimal living thing, which meets the criterion: (a) is auto-catalytic, (b) completes one thermodynamic work cycle, (c) has a membrane—push their way into novelty, molecular, morphological, behavioral, and organizational novelty. 
The following are related quotes:
“There seem to be ‘laws’ [of] social systems that have at least something of the character of natural physical laws, in that they do not yield easily to planned and arbitrary interventions. Over the past several decades, social, economic and political scientists have begun a dialogue with physical and biological scientists to try to discover whether there is truly a ‘physics of society’, and if so, what its laws and principles are. In particular, they have begun to regard complex modes of human activity as collections of many interacting ‘agents’—somewhat analogous to a fluid of interacting atoms or molecules, but within which there is scope for decision-making, learning and adaptation.”— Philip Ball (2003), “The Physics of Society” 
● Active Brownian agent | Browning motion
● Homo economicus
● Vilfredo Pareto
1. Nelson, Alan. (1992). “Human Molecules”, in: Post-Popperian Methodology of Economics, ch. 3, pgs. 113-33) by Neil De Marchi. Kluwer Academic Press.
2. (a) Anon. (1999). “Our Non-Ergodic Universe”, Evolutionary Theory: An Esalen Invitational Conference, Nov. 14-19, Esalenctr.org.
(b) Kauffman, Stuart. (2002). Investigations (adjacent possible, 51+ pgs). Oxford University Press.
3. Ball, Philip. (2003). “The Physics of Society”, A talk Delivered at the London School of Economics, March.
4. Iorga, Ana. (2012). “Homo Economicus: Chronicle of a Death Foretold”, AgencyPost.com, Jun 19.
5. (a) Merriam-Webster Collegiate Dictionary, 2000.
(b) Hmolscience elaboration.
● Mirowski, Philip. (1997). “Machine Dreams: Economic Agents as Cyborgs”, pgs. 13-40, in New Economics and its History by John Davis. Duke University Press.
● Mirowski, Philip. (2002). Machine Dreams: Economics Becomes a Cyborg Science (draft pdf) (agent, 72+ pgs). Cambridge University Press.
● Floridi, Luciano and Sanders, J.W. (2004). “On the Morality of Artificial Agents” (Abstract), Minds and Machines. Springer.
● Ksenzhek, Octavian S. (2007). Money: Virtual Energy: Economy Through the Prism of Thermodynamics (agent, 3+ pgs; agents, 3+ pgs). Universal Publishers.
● Agent – Wikipedia.