Defining the indefinable. The method for knowing the existence of God according to Thomas Aquinas and the ‘unum’ argument of Anselm of Canterbury

Abstract

This paper sets out to defend the thesis of the little remarked similarity that exists in the writings of Thomas Aquinas and Anselm of Canterbury, through the analysis of the notion and use of nominal definition in natural or philosophical theology. In the third article of the sixth question of the Exposition of Boethius' “de Trinitate”, Thomas introduces the distinction between nominal definition and real definition in order to apply them to the knowledge of God. His argument is that, since he cannot be known directly, nor can he be given a real definition for this very reason, one must start from a nominal definition of what God is. Such a definition presupposes some knowledge of God, but negative, through the attributes of his effects and by eminence. The nominal definition of God serves, then, as the presupposition of the demonstration. In the Proslogion and in his Reply to the criticisms raised against him by Gaunilo in his time, Anselm agrees with this distinction of definitions, and that God cannot be known directly, unless by causation, negation or eminence. But, unlike Thomas, he sees that the nominal definition of God is sufficient to produce a ‘single argument which would need for its proof no other than itself, and which would suffice alone to assure us that God is in a true way’, since this is self-evident to us. This comparison also helps to understand Thomas‘ specific criticisms of Anselm's argument, notably that which denies that the knowledge of God is self-evident to us (’quoad nos").