Franz Dietrich (Paris) gives a talk at the MCMP Colloquium (26 June, 2013) titled "Reason-based rationalization". Abstract: In economics and other social sciences, choice behaviour is usually explained (or rationalized) by means of fixed preferences which the agent seeks to satisfy. This model of choice not only conflicts with observed behaviour, but also fails to account for important aspects such as the relevance of the choice context and the possibility of preference change. These problems stem from a failure to (i) address the reasons underlying choices and preferences, and (ii) distinguish between the modeller's and the agent's perception of the decision problem. We introduce a new way to rationalize an agent's choice behaviour. A 'reason-based' rationalization explains behaviour in terms of the motivationally salient properties of the options and/or the context. Such rationalizations allow us to explain several non-standard choice patterns, model the agent's subjective perception of alternatives, and predict choices in novel, not yet observed contexts. We show how behaviour can reveal which properties are motivationally salient. We determine the behavioural implications of two kinds of context-dependence: context-regarding motivation (i.e., concern for the context) and context-variant motivation (i.e., psychological instability).