“To argue from authority is most appropriate to this science; for its principles rest on revelation, and it is proper to credit the authority of those to whom the revelation was made. Nor does this derogate from the dignity of this science; for although proof from authority based on human reason may be weak, yet proof from authority based on divine revelation is most effective. “Yet sacred science also makes use of human reason; not indeed to prove the Faith, because this would take away the merit of believing; but to make manifest other things which may be treated in this science. For since grace does not annul nature, but perfects it, natural reason should serve faith, even as the natural inclination conforms itself to love (_caritas_). Hence sacred science uses the philosophers also as authority, where they were able to know the truth through natural reason. It uses authorities of this kind as extraneous arguments having probability. But it uses the authorities of the canonical Scriptures arguing from its own premises and with certainty. And it uses the authorities of other doctors of the Church, as arguing upon its own ground, yet only with probability. For our faith rests upon the revelation made to the Apostles and Prophets, who wrote the canonical books; and not upon the revelation, if there was any, made to other doctors.”[421] Mediaeval thought was beset behind and before by the compulsion of its conditions. Its mighty antecedents lived in it, and wrought as moulding forces. Well we know them, two in number, the one, of course, the antique philosophy; the other, again of course, the dogmatic Christian Faith, itself shot through and through with antique metaphysics, in the terms of which it had been formulated. These two, very dual and yet joined, antagonistic and again united, constituted the form-giving principles of mediaeval thinking. They were, speaking in scholastic phrase, the substantial as well as accidental forms of mediaeval theology, philosophy, and knowledge. Which means that they set the lines of mediaeval theology or philosophy, and caused the one and the other to be what it became, rather than something else; and also that they supplied the knowledge which mediaeval men laboured to acquire, and attempted to adjust their thinking to. Thus, through the twelfth, the thirteenth, and the fourteenth centuries, they remained the inworking formal causes of mediaeval thought; while, on the other hand, the moving and efficient causes (still speaking in scholastic-Aristotelian phrase) were the human impulses which those formal causes moulded, or indeed suggested, and the faculties which they trained.