On The Holy Trinity by St. Augustine
Chapter 9.--The Three Persons Not Properly So Called [in a Human Sense].
But because with us the usage has already obtained, that by essence we understand the same thing which is understood by substance; we do not dare to say one essence, three substances, but one essence or substance and three persons: as many writers in Latin, who treat of these things, and are of authority, have said, in that they could not find any other more suitable way by which to enunciate in words that which they understood without words. For, in truth, as the Father is not the Son, and the Son is not the Father, and that Holy Spirit who is also called the gift of God is neither the Father nor the Son, certainly they are three. And so it is said plurally, |I and my Father are one.| For He has not said, |is one,| as the Sabellians say; but, |are one.| Yet, when the question is asked, What three? human language labors altogether under great poverty of speech. The answer, however, is given, three |persons,| not that it might be [completely] spoken, but that it might not be left [wholly] unspoken.