Philosophie de la finance : l’exemple de l’efficacité informationnelle d’un marché

This chapter aims at introducing the concept of « Informationally Efficient Market » with the intent to set in the frame of economic philosophy some notions actually present in contemporary finance theory and some issues still at stake to our days, both in the financial industry that attempts at making the best use of these concepts and in the general public that undergo a seemingly boundless financialization of their lives. The first section deals with a philosophical history of such ideas. The issue dealt with is how to find a scientific statement of informational efficiency, while putting forth some historical long-run landmarks related to this concept and while displaying its many-faceted variations in meaning. We shall in particular insist on the definitional change that occurred in the 1980s. The second section deals with the epistemology, again in relationship with the same idea. It is considered with regard to its non-refutability, using Hempel's criterion, and while we describe necessary auxiliary hypotheses that underlie its main theoretical assessment – much in the sense of Lakatos' famous notion of a "research-program". As an example, we discuss the case of speculative bubbles, leading to the issue whether "conventionalist stratagems" (Popper) do exist. To conclude, we shall reflect upon the predominance of a theoretical stand about information efficiency over all the rest and the way in which this assessment gradually changed the world of finance.