Objection 1. It would seem that if we abstract the Personality by our mind, the Nature cannot assume. For it was said above (Article 1) that it belongs to the Nature to assume by reason of the Person. But what belongs to one by reason of another cannot belong to it if the other is removed; as a body, which is visible by reason of color, without color cannot be seen. Hence if the Personality be mentally abstracted, the Nature cannot assume.
Objection 2. Further, assumption implies the term of union, as was said above (Article 1). But the union cannot take place in the nature, but only in the Person. Therefore, if the Personality be abstracted, the Divine Nature cannot assume.
Objection 3. Further, it has been said above (I, 40, 3) that in the Godhead if the Personality is abstracted, nothing remains. But the one who assumes is something. Therefore, if the Personality is abstracted, the Divine Nature cannot assume.
On the contrary, In the Godhead Personality signifies a personal property; and this is threefold, viz. Paternity, Filiation and Procession, as was said above (I, 30, 2). Now if we mentally abstract these, there still remains the omnipotence of God, by which the Incarnation was wrought, as the angel says (Luke 1:37): "No word shall be impossible with God." Therefore it seems that if the Personality be removed, the Divine Nature can still assume.
I answer that, The intellect stands in two ways towards God. First, to know God as He is, and in this manner it is impossible for the intellect to circumscribe something in God and leave the rest, for all that is in God is one, except the distinction of Persons; and as regards these, if one is removed the other is taken away, since they are distinguished by relations only which must be together at the same time. Secondly, the intellect stands towards God, not indeed as knowing God as He is, but in its own way, i.e. understanding manifoldly and separately what in God is one: and in this way our intellect can understand the Divine goodness and wisdom, and the like, which are called essential attributes, without understanding Paternity or Filiation, which are called Personalities. And hence if we abstract Personality by our intellect, we may still understand the Nature assuming.
Reply to Objection 1. Because in God "what is," and "whereby it is," are one, if any one of the things which are attributed to God in the abstract is considered in itself, abstracted from all else, it will still be something subsisting, and consequently a Person, since it is an intellectual nature. Hence just as we now say three Persons, on account of holding three personal properties, so likewise if we mentally exclude the personal properties there will still remain in our thought the Divine Nature as subsisting and as a Person. And in this way It may be understood to assume human nature by reason of Its subsistence or Personality.
Reply to Objection 2. Even if the personal properties of the three Persons are abstracted by our mind, nevertheless there will remain in our thoughts the one Personality of God, as the Jews consider. And the assumption can be terminated in It, as we now say it is terminated in the Person of the Word.
Reply to Objection 3. If we mentally abstract the Personality, it is said that nothing remains by way of resolution, i.e. as if the subject of the relation and the relation itself were distinct because all we can think of in God is considered as a subsisting suppositum. However, some of the things predicated of God can be understood without others, not by way of resolution, but by the way mentioned above.
Source: Summa Theologica (New Advent)