In Articles 1 through 8 of Summa Theologica I Question 3, Aquinas offers a series of philosophical arguments in favor of DDS in an apophatic way. In that regard, he addresses eight questions: whether God is a body, whether God is composed of form and matter, whether God is the same as His essence or nature, whether God essence and existence are the same in God, whether God is contained in a genus, whether there are accidents in God, whether God is altogether simple, and whether God enters into the composition of other things.
The Thomistic doctrine of simplicity can be summarized as that everything in God is God. In this respect, there are some aspects to mention. From the very beginning in Summa Theologica I.3, a.1, Aquinas suggests that the language one should use to describe God must be analogical — that is, we should understand religious language as a comparison between spiritual and corporeal realities. Thus, for Aquinas, the Doctrine of the Analogy is the best way to approach religious language or God-talk, so he seems to reject any univocal approach when he refers to God. Also, Aquinas’s approach to divine attributes is strongly shaped by the idea that God is pure act and that anything else is a complexity of act and potentiality. The importance of these two principles is that they permeate the whole Thomistic discussion of DDS.
When Aquinas moves on to Article 2 regarding of whether God is a complexity of form and matter, he makes clear that this question should be answered negatively. However, Aquinas argues that not all forms are composite of matter. God, for example, is a self-subsisting form (ipsum esse subsistens). With this claim, Aquinas strengthens his overall argument in favor of DDS by not locating God and creatures in a continuum of being.
The discussion in Articles 3 and 6 of Summa Theologica I.3 constitutes perhaps the central debate regarding divine simplicity. In Article 3, Aquinas discusses whether God is the same as His essence or nature. Aquinas rejects the notion that God is a subject having an essence as it is observed in the created order. Aquinas claims that as God is not a composition of form and matter, so the observed principle in creatures does not apply to Him. For God his form coincides with His nature. This makes that God is called to be both life and a living thing. With this in mind, Aquinas argues that anything predicated of God is God, a central core of DDS. Similar to Article 3, Aquinas reinforces his argument in favor of divine simplicity in his Article 4. He analyzes whether God essence and existence are the same in God. In this respect, Aquinas offers a positive answer. Aquinas argues God is identical to his existence and essence. In Article 6, Aquinas argues that God cannot have any accidental properties because he is the ultimate cause of things and there is not a thing in Him which it has been caused. This Article is of significance to our study in the sense that modern philosophy in the analytic tradition has usually understood the term ‘accidents’ in a modern sense, leading to a misinterpretation of Aquinas’s understanding of divine simplicity.
Overall, the importance of Question 3 and the Articles mentioned above is that Aquinas connects the doctrine of simplicity to other attributes of God, including God’s aseity and sovereignty, besides offering a solid argument against a pantheist/panentheistic understanding of God. As one observes, the medieval terms and concepts are a delicate issue with which the philosopher or theologian must deal. It is easy that such an issue might lead to serious confusion when interpreting Aquinas.