Organisms change their shape and behavior during ontogenesis in response to incentives—what biologists call “phenotypic plasticity” or what is called here more specifically “behavioral plasticity.” Such plasticity is usually in the direction of enhancing welfare or fitness. In light of basic concepts in economics, such behavioral plasticity is nothing but rationality. Such rationality is not limited to organisms with neural systems. It also characterizes brainless organisms such as plants, fungi, and unicellular organisms. The gist of the article is the distinction (...) between rationality and intelligence. Whereas rationality is ubiquitous in all organisms, intelligence varies in degrees depending on division of labor that may involve the evolution of a neural system. This article aims to defend the universality of rationality—the Organismus oeconomicus hypothesis. It argues that neither the notion of bounded rationality of behavioral economics nor Herbert Simon’s “procedural rationality” can ultimately undermine the Organismus oeconomicus hypothesis. (shrink)