Since distinction in the Godhead is accounted for by relations that are not accidental but are subsistent, and since among beings subsisting in an intellectual nature personal distinction is discerned, it necessarily follows that personal distinction in God is constituted by the relations in question. Therefore the Father and the Son and the Holy Spirit are three persons, and also three hypostases, since hypostasis means something that is subsistent and complete.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Personal Distinction in God Through the Relations, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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