In a 1950 paper, Paul Samuelson wrote:
The most rational man I ever met, whom I shall call Ysidro [when] told that he did not satisfy all of the v. Neumann-Morgenstern axioms, [..] replied that he thought it more rational to satisfy his preferences and let the axioms satisfy themselves.
This introduced in the extensive correspondence between Samuelson, Savage, Marschak, Baumol, and Friedman the idea of the “Ysidro Man” or “Ysidro functions.” In the letters (but not in published print) Samuelson also introduced his mother – as the non-economist acting on her common sense. Thus, for instance, Marschak would discuss with Baumol how best to axiomatize the behavior of Samuelson’s mother.
These archetypes are more commonly labelled homo economicus and homo sapiens – very dull terms indeed (and why the Latin anyway?). So, from now on, when we talk about the homo economicus let’s instead ask: What would Ysidro do? And when criticizing that economic conception as unrealistic, let’s do that by referring to what Samuelson’s mother would do.