Tribute to Jean - Yves Jaffray

It is very rare to find in a single person both the qualities of a remarkable scientific mind and of a wonderful human being. With this tribute to Jean-Yves Jaffray, we hope to convince the reader of his outstanding creativity and vision, of the coherence of his work, and of its relevance for some topics in decision theory that are currently under lively debate. As a scientist, Jean-Yves Jaffray can be characterized by one main insight and one main concern. His main insight is that a sound decision theory must explicitly use all the information available to the decision maker. This information about events must further be treated in a strictly objective manner. In the models he proposed as a result, objective information can be disentangled from subjective attitudes with respect to this information. For that purpose, before asking how to represent preferences, one must wonder how to treat and represent the given information. Jean-Yves’ main concern in designing his models is that they must be tractable, implementable and testable. This leads him to emphasize the simplicity of the models he puts forward, including the way the arrival of new information is modeled, and to develop experiments to test them. This adherence to objectivity together with his concern for implementable models fits well with Jean-Yves’ applied work in statistics and computer science that we will not review here. After some words on his early contributions, we will discuss the way he addressed different questions linked to Decision Theory : How to describe information (or lack of information) on events? How to model decisions in this framework? How to evaluate decisions? How to update in the presence of new