"Did we set ourselves to the task of proving by argument or by authority that we are not wrong in believing in our own existence or that of an external world, or did we attempt to establish the trustworthiness of our faculties by resolving it into the veracity of God, our effort must needs be as abortive as it is superfluous, since it involves the necessity not only of proving the fact, but of _proving the proof itself_, and that, too, by the aid of the very faculties whose trustworthiness is in question!"