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cf., SCG4.C26, QDePot.Q9.A9
At this point, we reach the end of our analysis of the notion of the "procession" in God. Here, we are able to explain a truth of the Catholic faith that many Catholic theologians of former ages were not able to express. Why are there only two processions and not an infinite regress?
The objector asks, "if God is infinite, why does he not keep communicating the Divinity to an infinite number of persons?"
The reason is expressed by St. Thomas (a reason which one should already be able to gather from what we have discussed above), "the divine processions can be derived only from the actions which remain within the agent. In a nature which is intellectual, and in the divine nature, these actions are two, the acts of intelligence and of will."
This question brings us to make a profound distinction in Trinitarian theology. For, the processions we have spoken of cannot be absolute, i.e., predicated of the Divine nature as a whole, but, must be relative, i.e., possessed.Â
For, otherwise, we would have the terminus (*ad quem*) of the procession proceeding from itself.Â
Thus, we distinguish between the principium quo and the principium quod.
The principium quod is the principle from which another proceeds. For our example, let's take heat proceeding from fire to warm a pot of water.Â
The terminus a quo of the procession (i.e., the principium quod) is the fire. The terminus ad quem of the procession is the pot of water. The heat proceeds from the fire to the water.Â
On the other hand, we may ask a completely separate question, one we have been asking thus far, what is the inherent power by which the heat proceeds from the fire to the water. This is the question of the principium quo. The principle by (through) which the heat passes from the water to the water is the inherent form of heat present in the fire. The principium quo is always participatory. That which is communicated and that by which it is communicated "mirror" each other. Thus, the father as principium quod communicates humanity to his son by his own humanity as principium quo.Â
Now, how does this map on in God? Let's take the instance of the Word. The Word is begotten by way of intellection. Yet, we have established that this is not simply by way of the absolute understanding that is shared by the three persons. Rather, it is a relative enunciation that is spoken by the Father. Thus, the principium quo is the intellect as it is possessed by the Father. This easily resolves our question on the principium quod of the procession, which is the Father.Â
In a similar way, the principium quod of the spiration of the Spirit is the Father and the Son, and the principium quo is the will as possessed by the Father and the Son.
Respondeo: The divine processions can be derived only from the actions which remain within the agent. In a nature which is intellectual, and in the divine nature, these actions are two, the acts of intelligence and of will. The act of sensation, which also appears to be an operation within the agent, takes place outside the intellectual nature, nor can it be reckoned as wholly removed from the sphere of external actions; for the act of sensation is perfected by the action of the sensible object upon sense. It follows that no other procession is possible in God but the procession of the Word, and of Love.