Many commentators of Kant assume that the Refutation of Idealism is directed against a radical sceptic whose sole claim is immediate knowledge of his own representations in inner experience, including, to some extent, their temporal order. Accordingly, the Refutation is viewed as an attempt to establish that the perception of external objects is a prerequisite of knowing the temporal order of our representations. Here it will be argued that this minimal claim has to be supplemented by the proposition that the idealist is aware of himself as a substance in time, if Kant’s Refutation is to be reconstructed as a successful argument. In addition, the view will be defended that the argument is not appropriate to decide the questio facti whether there are external objects or not. It rather establishes that the determination of the self as a substance in time is conceptually dependent on the perception of external objects. As a consequence, the argument does not preclude the possibility that some people are brains in a vat, as Q. Cassam justly remarked.