Objection 1. It would seem that there are more than two processions in God. As knowledge and will are attributed to God, so is power. Therefore, if two processions exist in God, of intellect and will, it seems that there must also be a third procession of power.
Objection 2. Further, goodness seems to be the greatest principle of procession, since goodness is diffusive of itself. Therefore there must be a procession of goodness in God.
Objection 3. Further, in God there is greater power of fecundity than in us. But in us there is not only one procession of the word, but there are many: for in us from one word proceeds another; and also from one love proceeds another. Therefore in God there are more than two processions.
On the contrary, In God there are not more than two who proceed--the Son and the Holy Ghost. Therefore there are in Him but two processions.
I answer that, 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.
Reply to Objection 1. Power is the principle whereby one thing acts on another. Hence it is that external action points to power. Thus the divine power does not imply the procession of a divine person; but is indicated by the procession therefrom of creatures.
Reply to Objection 2. As Boethius says (De Hebdom.), goodness belongs to the essence and not to the operation, unless considered as the object of the will.
Thus, as the divine processions must be denominated from certain actions; no other processions can be understood in God according to goodness and the like attributes except those of the Word and of love, according as God understands and loves His own essence, truth and goodness.
Reply to Objection 3. As above explained (14, 5; 19, 5), God understands all things by one simple act; and by one act also He wills all things. Hence there cannot exist in Him a procession of Word from Word, nor of Love from Love: for there is in Him only one perfect Word, and one perfect Love; thereby being manifested His perfect fecundity.
Source: Summa Theologica (New Advent)