On The Holy Trinity by St. Augustine
Chapter 12.--In Relative Things that are Reciprocal, Names are Sometimes Wanting.
13. Neither ought it to influence us -- since we have said that the Holy Spirit is so called relatively, not the Trinity itself, but He who is in the Trinity -- that the designation of Him to whom He is referred, does not seem to answer in turn to His designation. For we cannot, as we say the servant of a master, and the master of a servant, the son of a father and the father of a son, so also say here -- because these things are said relatively. For we speak of the Holy Spirit of the Father; but, on the other hand, we do not speak of the Father of the Holy Spirit, lest the Holy Spirit should be understood to be His Son. So also we speak of the Holy Spirit of the Son; but we do not speak of the Son of the Holy Spirit, lest the Holy Spirit be understood to be His Father. For it is the case in many relatives, that no designation is to be found by which those things which bear relation to each other may [in name] mutually correspond to each other. For what is more clearly spoken relatively than the word earnest? Since it is referred to that of which it is an earnest, and an earnest is always an earnest of something. Can we then, as we say, the earnest of the Father and of the Son, say in turn, the Father of the earnest or the Son of the earnest? But, on the other hand, when we say the gift of the Father and of the Son, we cannot indeed say the Father of the gift, or the Son of the gift; but that these may correspond mutually to each other, we say the gift of the giver and the giver of the gift; because here a word in use may be found, there it cannot.