**Abstract**For a host of compelling methodological reasons,

*homo oeconomicus*has to be replaced. This is consensus, the open question is how this could be accomplished. What is required first is the separation of the formal foundations into a structural and a behavioral part. This paper introduces the propensity function as general formalization of Economic Man/Woman. The propensity function is a compact formal expression of random, semi-random, and deterministic behavioral assumptions. It is shown how, in a random environment, target-oriented behavior produces stochastic stability and optimality in the product market. With

*homo oeconomicus*the conception of simultaneous equilibrium, too, vanishes.

For the complete set of foundational equations — structural axioms and behavioral propensity function — see here