Nature of the Relations Whereby the Father, the Son, and the Holy Spirit Are Distinguished

The relations by which the Father, the Son, and the Holy Spirit are distinguished from one another are real relations, and not merely mental relations. Those relations are purely mental which do not correspond to anything found in the nature of things, but depend on intellectual apprehension alone. Thus right and left in a stone are not real relations, but only mental relations; they do not correspond to any real disposition present in the stone, but exist only in the mind of one who apprehends the stone as left, because it is, for instance, to the left of some animal. On the other hand, left and right in an animal are real relations, because they correspond to certain dispositions found in definite parts of the animal. Accordingly, since the relations whereby the Father and the Son and the Holy Spirit are distinguished really exist in God, the relations in question must be real relations, and are not merely mental relations.

Reference

St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Nature of the Relations Whereby the Father, the Son, and the Holy Spirit Are Distinguished, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947

All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).

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