According to St. Thomas, the ontological foundation for the incomprehensibility of what God is (his quiddity or essence) is indeed divine simplicity. However, incomprehensibility is due, according to Thomas, not to the simplicity of the essence as such. It is due rather to the impossibility of the divine essence existing (actus essendi) in the abstract. In other words, the nature of God is such that there can be no idea formed of it.
A fundamental clarification must be made regarding the meaning of Thomas’s distinction between essence and existence (esse) or act of existing (actus essendi). The esse is not an accident of the essence. It is not an essential attribute. It is outside the essence. In other words, that a thing is adds nothing to what it is. An essence does not change with respect to its essence (what it is) upon the addition of esse (that it is). For example, if I asked a friend to describe his dog to me, he would probably not describe it as brown, furry, tall, and existing. The last attribute is clearly not an attribute in the sense of constituting the essence. It is not what Aristotle called a predicamental. It is rather presupposed to the other predicates, which however do not loose any of their conceptual content if it turns out that my friend’s dog is imaginary.
With this in mind, it becomes clearer as to what the meaning of Thomas’s account of divine incomprehensibility really comes down to.
God, according to Thomas, does not possess the mode of existence of creatures. This mode includes the various kinds of composition characteristic of created being (STh I, q. 3). He includes in his list of compositions characteristic of created being the composition of essence and existence (esse). It is this composition which makes conceptual knowledge possible.
The intellect, according to Thomas, conceives within itself the essence of the object of its intuition. It literally becomes its object in an intellectual way. Thus the knower and the thing known are united with respect to the essence which forms each: outside the mind in matter, inside the mind in the intellect itself. This cognizing operation presupposes the distinction between essence and esse or a particular object’s act-of-existing. When I come to know the object before me, I have in a sense acquired its essence, although its own act-of-existing remains outside conceptualization.
The ultimate reason then that Thomas gives for the impossibility of unaided knowledge of the divine essence is the impossibility of the intellect abstracting from God’s act-of-existing (esse) a rational form. In other words, one cannot have an idea of God. There is no idea, as such, of the divine essence. There is only and can only be “the” divine nature actually existing outside the mind. God’s esse, in other words, does not give up its secrets to the philosophers. This is because there can be no distinction made, as there is no objective foundation for the distinction between, the divine essence and the divine esse.
It also follows from this that God is not directly the subject of philosophy. Philosophy, and particularly Platonic philosophy, is a knowledge based upon essences. It seeks to know not the esse of things (and in that sense not “things” at all) but rather the purely rational order of essences.
Ultimately, for Thomas, the reason for the incomprehensibility of the divine essence is not the same reason for the incomprehensibility of the Plotinian ‘one’. Whereas the latter is inconceivable due to the simplicity of its pure essence, the former is incomprehensible due to its irreducible concreteness, realness, subsistence.