Amphithéâtre Marguerite de Navarre, Site Marcelin Berthelot
Open to all
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1. We began by presenting The Magnitude of the Skeptical Challenge , emphasizing the unnatural and natural character of doubt on the one hand, and the magnitude of the challenge as it presents itself in particular in the form of Humean skepticism on the other. This was followed by a survey of the possible degrees and varieties of doubt, before presenting the new figures of skepticism in contemporary philosophy, highlighting the problems of definition and unity of skepticism; its ancient and modern versions, and in what form these variants are taken up in the figures of contemporary skepticism. We began by recalling the four attitudes we can adopt with regard to our beliefs: 1) judging them to be true and approving them: "It's true that we know a lot of things! "I know (dogmatism of all kinds); 2) I know, but only in a probable way (Carneades' version: probabilism); 3) it's impossible for me to determine whether I know or don't know, so I suspend my assent (pyrrhonism); 4) all things considered, I know nothing (dogmatic scepticism).

2. We then presented the two classic forms of skeptical challenge to knowledge: Agrippa's challenge and the Cartesian challenge.

a) The Cartesian challenge and the argument from ignorance. A skeptical hypothesis is a possibility of error that is incompatible with the knowledge we believe we have, but which we are also unable to measure as normal or not: this is the Cartesian or Putnamian scenario (H. Putnam, 1984). Hence the formulation of the skeptical enigma or "paradox": Sc1: I can't know that the skeptical hypotheses (e.g. that I'm dreaming or that I'm a brain in a vat (CC)) are false; Sc2: If I don't know that the skeptical hypotheses are false, I don't know very much; Sc3: So I don't know very much. (But I do know a lot: the conclusion is therefore "paradoxical"). The paradox can also be stated in the form of the "argument from ignorance" (DeRose, 1995: 1. I don't know that non-H; 2. If I don't know that non-H, then I don't know that O; 3. Therefore, I don't know that O). There areother possible formulations: to know that P, I must eliminate all possibilities that non-P; I can't eliminate all possibilities that non-P; therefore, I don't know that P. Or: if I know that P, then I have no reason to doubt that P; X is a good reason to doubt that P; therefore, I don't know that P.

b) Agrippa's challenge. We recall the definition of knowledge (K) given from Plato: (K): S knows that P if and only if: a) S believes that P; b) P is true; c) S is justified in believing that P. And what have been the main responses proposed to this aporetic definition of knowledge, either in the internalist versions (foundationism - Cartesian tree - and coherentism - Neurath's raft) or in the externalist versions (Dretske, Nozick, Goldman) of justification. We also recall the counterfactual formulation given by Nozick (1981) (based on the idea of a truth-sensitivity condition that we "follow through"): KT: S knows that P if and only if: 1) P is true; 2) S believes that P; 3) if P were not true, S would not believe that P; 4) if, in counterfactual situations, P were always true, S would always believe that P.