Abstract. Nowadays, it seems almost universally presumed that the fundamental characteristic of homo oeconomicus is his rationality. We analyze the role played by the concept of rationality in economic theory, and demonstrate that it is necessarily constrained to be an essentially contentless notion. We show that the main body of economic theory is firmly grounded, and that some contrasting approaches to rationality, although leading to heated debates and vivid confusion, have no fundamental significance for economics. With a refreshed view on the essence of economics, we argue that the principles of economic theory form an essential methodological guide for the emergent line of research based on the use of so-called 'evolutive' models.