Substantia is in this passage used as the equivalent of Person. The word was used by Tertullian in the sense of οὐσία, and this early Latin use of the word is the use which eventually prevailed. The meaning of the word in Hilary is influenced by its philological equivalent in Greek. At the beginning of the fourth century ὑπόστασις was used in the same sense as οὐσία. The latter word meant ‘reality,’ the former word ‘the basis of existence.’ Athanasius, however, began the practice of restricting ὑπόστασις to the divine Persons. Hilary consequently here uses substantia in this new sense of the word ὑπόστασις. The Alexandrine Council of 362 sanctioned as allowable the use of ὑπόστασις in the sense of Person, and by the end of the century the old usage practically disappeared.