Brown’s rebellion against Reid early in the century, in the interest of a universal physical causation or association, has its parallel in Ferrier’s revolt, in the middle of the century, against the Hamiltonian Reid, in the interest of abstract metaphysics as opposed to uncriticised common sense. In the name of philosophy he excludes from philosophy all except necessary truths of abstract reason; neglecting, as beneath its regard, the world of change, in which Reid’s mixed and practical reason, or Common Sense, had been offered as final guide. The office of philosophy, according to Ferrier, is among the eternal truths, which alone can be absolutely demonstrated, and which relieve philosophy from ‘the oversight of popular opinion and the errors of psychological science,’ which had been unworthily dignified as the final test of truth. The natural beliefs of mankind, instead of being worshipped as divine, are banished in Ferrier’s philosophy, on the ground that their self-contradictoriness is demonstrable: the business of the philosopher, accordingly, is to substitute ‘reasonable thinking’ for ‘common sense.’ The ‘_raison impuissante_,’ emphasised by Hamilton—the ignorance in which Hamilton revels—is not allowed by Ferrier to be ignorance at all; for ‘man cannot be said to be ignorant of self-contradictions that can be knowledge for no mind, human or divine.’ Independent or unperceived matter is not merely hid from man’s knowledge on account of his ‘_raison impuissante_’; it is hid from all intelligence, because inconsistent with the _necessarily_ mind-dependent essence of Being. Pure reason does not need to be finally supplemented by practical principles of common sense. It is able to shift for itself without this surrender. The conciliation of common sense thinking and philosophy is accomplished by the submission of common sense to universal compulsory reason. Opinion must submit to demonstration, instead of demonstration, intelligible only by the few, having to make way for the undemonstrable dogmas of the unreflecting.