From St. Peter Damian's Letter 91 to Patriarch Constantine Lichoudes of Constantinople:
But if one should ask, "Since the Son is of the substance of the Father, and the Holy Spirit is also of the substance of the Father, why is one the Son and the other not also the Son?" it would not be inconsistent to reply as follows. The Son is from the Father, the Holy Spirit is from the Father, but the former is begotten, while the latter proceeds; and therefore the former is the Son of the Father, from whom he is begotten, while the latter is the Spirit of both because he proceeds from both. Nevertheless, this begetting and procession are not only ineffable but also totally incomprehensible. But in those things which we are unable to penetrate with the power of our mind, we apply a sure faith to those through whom the Holy Spirit has spoken, just as if the matter lay clearly before our eyes. And even though these hidden mysteries of profound depth are unknown to us, still we are not in doubt about what the Lord has spoken, we are also not uncertain about what is found in the pronouncements of the prophets.
[Peter Damian, Letters 91-120, Blum, tr., The Fathers of the Church: Medieval Continuation, CUA Press (Washington DC: 1998) p. 11.]