Abstract
In this paper, I aim at demonstrating that the break point between neoclassical and behavioural economics lays first on the way rationality is defined. The concept was perfectly unified through maximisation. Simon elaborates the initial version of behavioural economics criticising neoclassical concept of rationality and suggesting alternative reasoning modes are followed by agent. Even if Kahneman and Tversky tend to supply a typology of reasoning modes, they finally reunify them in Cumulative Prospect Theory. So the reunification of rationality is the cornerstone of the distinction between old and new behavioural economics and allows as well to explain the contemporary debates between Kahneman and Tversky and Gigerenzer within the latter one.