In Vatol's circumstances, that presumption is defeated - by his knowledge that his world contains envatted people. Having an experience that seems to suggest that p, according to Yitzchak, provides you with a defeasible presumption that p is the case. Lev thinks that our situation is no different to Vatol's.

Vatol, they all agree, has reason to doubt. They imagine a character, Vatol, in just that situation. All of the protagonists agree that, if we knew that we lived in a world in which some people were envatted (i.e., existed only as brains in vats), then we would have reason to doubt our so-called external reality.

The real philosophical action takes place between his two very different, ex-students-cum-colleagues, Yitzchak and Lev. Daniel plays the role of an honest arbiter. Hirsch's three characters are Daniel, Yitzchak, and Lev. One is reminded of the literature of the Haskalah. If the Berkeley-Beckett-Brecht combination isn't sufficient to whet your appetite, Hirsch throws in more than a soupçon of Talmudic allusion, and a distinctively Jewish sense of humour. In this text, therefore, we find the cutting philosophical dialogue of a George Berkeley, a dash of the theatrical absurdism of a Samuel Beckett, and a hint of the knowing self-reference of a Bertold Brecht. At one point, the fourth-wall is broken, and the characters express their awareness that they are characters of Hirsch's creation. Hirsch's richly conceived characters have just stepped outside of the study-hall of a rabbinical seminary (a Yeshiva), to discuss epistemology in the bathroom. Hirsch's dialogue does just that, and with wonderful theatricality. This allows the reader to see to the heart of the issues that divide the protagonists. Instead, they present a real clash between diverse voices, forcing one another to refine their position in light of each other's criticisms. The best dialogues avoid pitting strong intellects against yes-men.

Shadow of doubt meaning