The Realism of Aquinas
In his book On Universals, Nicholas Wolterstorff examines Aquinas' theory of universals and concludes that it suffers from “a crucial and incurable ambiguity, or incoherence. ” Wolterstorff charges Aquinas with denying that there is anything two distinct things have in common while maintaining that forms or natures may be the forms or natures of several distinct things. Aquinas' resolution of the apparent contradiction is said to consist of the assertion that forms or natures exist outside the mind only as individualized in singulars and exist as universals only when they are abstracted from such things by some intellect. Universals are then concepts abstracted from distinct singulars which are similar but not identical. But in that case, Wolterstorff shows, it is not one nature which is abstracted but several and so-called universal concepts cease to have any foundation in reality. 1
The purpose of this paper is to re-examine Aquinas' theory of universals in order to assess the force of Wolterstorff 's criticisms of it. In the course of the re-examination I look at the different ways in which Aquinas describes natures, the ambiguity of terms such as 'universal' and 'similar' in his writings, and employ his often ignored theory of identity and distinction to show finally that, contra Wolterstorff, Aquinas does maintain that numerically distinct individuals of a kind share a nature and can thus be said to be identical with one another in a certain sense. Aquinas turns out to be almost as strong a realist as Duns Scotus.
According to Aquinas, the nature of a thing is its essence or quiddity, that which makes the thing the sort of thing it is. It is also said to be that through