The Issue
The idea of God as a Trinity has often led Jews and Muslims to accuse Christians of polytheism. Of course, the idea of three being one is a difficult and seemingly contradictory teaching. And this problem even persists with shared Biblical heritage supporting it. In the 26th verse of the Bible, God says “let Us make man in Our own image and likeness,” for example (Gen. 1:26). So, if the issue of the Trinity can cause a rift within Abrahamic religion, how much more of a stumbling block must it be to someone who is unsure of God in the first place?
But in fact, far from relying on dogmatism, the truth of the Trinity is logically discernible without requiring any recourse to scripture at all.
The Mind
In De Anima iii, Aristotle distinguishes the human mind from lesser animals by pointing out that humans think, while the animals merely perceive and instinctually react. Aristotle’s definition of, “thinking” is receiving or considering the essence of a thing. So, while the dog visually perceives this dog or that dog, the human can think about “dog-ness,” the very essence of dog. Crucially, Aristotle believes the essence of the mind is to become the essence of the object it considers. That is, when you think about a dog, your mind literally becomes dog-ness. So, to know something is to have what is known (the object) within the knower (by the mind’s becoming the object). Of course, the fidelity with which the mind conforms to the intelligible essence varies. A veterinarian’s knowledge of dog-ness is more perfect than that of a child, for example.
But Aristotle points out that this cannot be the only component of thought. Imagine a coffee cup moving from 70° to 75°. It is impossible for it to move itself from the one state to the other, as that would require it to be 75° while it was still 70°. Obviously, that’s impossible – something can’t be in two contradictory states at once. So, in order for the coffee to change temperature, it requires an actualizer – something external to it which moves it from one state to the other. Likewise, this passive part of the mind which transforms into different essences cannot transform itself. It requires something external to actualize the change. Aristotle describes a separate part of the mind which “makes all things.” That is, which acts upon the “passive” part of the mind to transform it into its object.
So, Aristotle describes two components of the mind: the active intellect and the passive intellect. The active intellect acts upon the passive intellect, turning potential knowledge into actual knowledge as light acts upon a prism, turning potential colors into actual colors. Now there is one final element of mind – the will, which is an appetite and actual power to act stemming from the mind. Aristotle’s argument for the will is very simple. If there was no such thing as will, then man would have no ability to resist his instincts. But man can, by virtue of his intellect, know better than his instincts and override them. So, the mind most certainly has the powers of intellect and of will.
God’s Nature
There are two things we must know about God before discussing the Trinity. These presuppose you understand that God is a self-evidently existing being. If that statement confuses you, please read this and then come back.
First, God is perfect. The definition of perfection is “to lack nothing.” For example, a “perfect” game of golf would be 18 holes-in-one, because a golf game could not be more complete. Anything imperfect (incomplete) has some part of itself which could be fulfilled by another, and is thus contingent. So God, by nature of being self-evident, must be perfect.
Second, God is a mind. God cannot be a material being, as materiality is always subject to external conditions, and a self-evident being cannot be subject to conditions. Now, knowledge has an inverse relationship with materiality. For example, a rock knows nothing. An animal experiences through sense images which are immaterial (free of the physical matter constituting them), but does not consciously “know” them. A human knows by understanding immaterial abstractions about these sense images. So, knowledge is precisely this layer of immateriality. And further still, knowledge is the only thing which can move material things while remaining immutable, as when the unchanging idea of ice cream causes your physical body to desire and retrieve ice cream. Consequently, an immaterial, immutable being with causal power must be a mind, and its complete immateriality means there is no sensorial nor physical constraint on its capacity for knowledge.
The Trinity
Now God is a mind, and thus God has the powers of active intellect, passive intellect, and will.
Recall that the passive intellect becomes its object. Since God is a mind, if God were to think about a dog, God Himself would become the essence of dog – but this is absurd, as this would transform God into not-God. Therefore, God’s only object of thought is Himself. But God is perfect, and depends on no external force to actualize His knowledge. So, His knowledge is always perfect. Therefore, He must know Himself perfectly; there is no distinction between Himself and His knowledge of self. But again, the passive intellect becomes the essence of its object. Therefore, because God knows Himself perfectly and is His own object of thought, His passive intellect is identical to His active intellect.
Now, as an aside, one might ask how God knows anything beside Himself if He only contemplates Himself? God, being the essential form of existence, sees and knows all existing things from within Himself. So the point is conceded, God doesn’t know anything beside Himself… because He’s all there is to know.
Next, recall that the will is the power which makes manifest the knowledge and judgment of the knower. Now a human may have a particular image in his mind that he wishes to paint, but by the impediment of a shaky hand, his will manifests his thought imperfectly. Or, a human may will something, but through a defect in his knowledge, fail to will it perfectly. God is perfect, so there is nothing which could possibly impede Him from knowing perfectly and manifesting His knowledge exactly as it is. Therefore, His knowledge is always made perfectly manifest, either by perfectly loving Himself or by perfectly loving creation as such into existence. Therefore, His will is perfectly identical to His passive intellect.
So God’s will is identical to His passive intellect, which is identical to His active intellect. Therefore, God’s mind is three distinct aspects of one unchanging substance. A trinity. Now in all minds, the active intellect begets the passive intellect by making objects knowable. Then the active and the passive beget the will by manifesting judgment. But in God, the passive intellect and the will are really all identical to their first principle. Or, as a Christian would say it: the Father (active intellect) begets the Son (passive intellect), and together they beget the Holy Spirit (will), while all three of these distinct persons remain substantially one.
These distinctions not only clarify the Nicene Creed, but clarify the fittingness of each person of God for their missions. The Father, being the active intellect, is most fit to create and providentially oversee all things. The Son, being passive intellect, is most fit to present the intelligible form of God in the person of Jesus. The Holy Spirit, being the will, is most fit to manifest the love of God in the faithful. Yet, each of the three remain totally and completely God. In the words of Christ, “whoever has seen Me has seen the Father” (John 14:9). This is literally true; Jesus is the intelligible essence of the Father.