Updated: Feb 2
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Our intellect, which is led to the knowledge of God from creatures, must consider God according to the mode derived from creatures. In considering any creature four points present themselves to us in due order. First, the thing itself taken absolutely is considered as a being. Second, it is considered as one. Third, its intrinsic power of operation and causality is considered. The fourth point of consideration embraces its relation to its effects. Hence this fourfold consideration comes to our mind in reference to God. (ST.I.Q39.A8.C)
The first analogy is in that we regard God “absolutely in His being.” (ST.I.Q39.A8.C.2) In this, the Father is “eternity,” the Son is “species,” and the Holy Spirit is “use.”
Eternity is “being without principle.” (Ibid.) The Father is like eternity in that He too is without principle, “principle without a principle.” (Ibid.) The Father is generated of none and is therefore like eternity.
The Son is like “species,” or, as St. Thomas more helpfully describes, “beauty.” Something is said to have “beauty” in that it meets three “requirements.” First, it has “integrity” (integritas) or “perfection” (perfectio). That is, the form of the thing does not depart from the idea in the mind of God, and become corrupted to being “ugly” “false” and “evil.” Second, “due proportion” (debita proportio) or “harmony” (consonantia). In another place, St. Thomas describes this idea of “due proportion” as “those [things] which please when seen.” (ST.I.Q5.A4.Rep1) Third, “clarity” (claritas) or brightness, as when we find bright colors to be beautiful.
First, the Son has “perfection” in that He is the express image of the Father in that He is image. The Son is related to the Father just as a beautiful thing is related to its prototype in the mind of God.
Second, the Son has “due proportion” in that He is the express image or the Father in that He is express. The Son shows forth the Father in perfect harmony, not lacking.
Third, the Son has “clarity” in that “Word,” not only shining forth it's own nature, but also illuminating the intellects of all.
The Holy Spirit is “use” in the sense of “enjoy” or “use joyfully” in that He is that “whereby the Father and the Son enjoy each other.” (ST.I.Q39.A8.C.6)
The second analogy is in considering God as “one.” In this, “unity” is as the Father, “equality” is as the Son, and “concord” or “union” is as the Holy Spirit, the three of which “imply unity, but in different ways.” (ST.I.Q39.A8.C.7) As Augustine says, “The Three are one, by reason of the Father; They are equal by reason of the Son; and are united by reason of the Holy Spirit.” (De Doctrina Christiana, Bk. 1, 5)
Unity is said “absolutely” and “in itself” without regard to another. For this reason, it is as the Father, for so also is the Father without regard to another and hypostatically “in Himself” and “or Himself.” As St. Thomas says, “Now unity is perceived at once in the person of the Father, even if by an impossible hypothesis, the other persons were removed. So the other persons derive their unity from the Father.” (ST.I.Q39.A8.C.8)
Equality is “unity as regards another.” (ST.I.Q39.A8.C.7) Something has “unity” with another when they have the same quantity as that other. This is said of the Son, not due to any quantity in the Godhead, but because of the sharing of the self-same Divine essence between the two. As St. Thomas says, “But if the other persons be removed, we do not find equality in the Father, but we find it as soon as we suppose the Son. So, all are equal by reason of the Son, not as if the Son were the principle of equality in the Father, but that, without the Son equal to the Father, the Father could not be called equal; because His equality is considered first in regard to the Son: for that the Holy Spirit is equal to the Father, is also from the Son.” (ST.I.Q39.A8.C.8)
Union is the “unity between two” and therefore is the Holy Spirit, “inasmuch as He proceeds from two.” (ST.I.Q39.A8.C.7) As St. Thomas says, “if the Holy Spirit, Who is the union of the two, be excluded, we cannot understand the oneness of the union between the Father and the Son. So all are connected by reason of the Holy Spirit; because given the Holy Spirit, we find whence the Father and the Son are said to be united.” (ST.I.Q39.A8.C.8)
The third analogy is in considering the power of God in causing amongst created things. The Father is power, the Son is Wisdom, and the Spirit is goodness, in that these are the three aspects of causality. Yet, we must be careful, for, if we are to consider each of these in the created realm, there is a great dissimilarity in them.
Power is the principle of a certain cause and is therefore like the Father, who is the principle of the Holy Trinity. Yet, in earthly Fathers, due to age, there is a lack of “power” which must be denied of the heavenly Father.
Wisdom is like the Son in that He is the Word of the Father. Yet, in earthly sons, due to lack of age, there is a lack of wisdom which must be denied of the heavenly Son.
Goodness is like the Spirit in that goodness is the object of love, which is the Holy Spirit.
The fourth analogy is in considering the relationship of God to His effects. From Whom refer to the Father; by Whom refers to the Son; in Whom refers to the Holy Spirit.
“From” can be taken in two senses. First, it can be taken in the sense of a material cause. That is, this chair is made from wood. God cannot be a material cause. Second, it can be taken in the sense of an efficient cause. That is, this chair is from the chair maker. In this sense, it is taken in the same way in which “power” was taken above.
“By” is taken in the sense, according to St. Thomas, of “the habitude of a form by which an agent works.” (ST.I.Q39.A8.C.10) In plain language, this refers to a certain “mental disposition” whereby someone works, differing from an instrument. Back to our chair analogy, the “instrument” whereby the builder works is a saw and hammer, the “habitude” is the art of chair building. In the later sense we take the Son.
“In” is taken in the sense of “containing.” God is said to contain in two ways. First, “by their similitudes,” that is, as existing in that God knows them. In this since, it is analogous to the Son. In the second sense, “things are contained in God forasmuch as He in His goodness preserves and governs them, by guiding them to a fitting end” which is analogous to the Spirit.