What estimation can be made of Photius? His logic is almost impeccable, but it is also completely self-contained. He did not know Augustine's arguments, nor did he reflect seriously on the Western tradition, which was a closed book to him. He allowed himself to be carried away by his own reasoning and came up with an analysis of the Filioque that his Latin contemporaries would not have recognized, even if they had been able to read his work. More seriously still, he never answered the positive challenge of the Filioque, which was to define how the Son and the Spirit were related. If the Spirit does not proceed from the Son, what is the nature of the connection between them?God Has Spoken, 669
Photius is one of the most controversial figures in church history. In the East, he is regarded as a saint who defended the theological tradition when it was under serious threat, but in the West he has traditionally been seen as the man mainly responsible for splitting the church into its Eastern and Western halves. What both sides agree on is that he was a brilliant scholar and a consummate politician who used his gifts for what he saw as the best interests of his church.God Has Spoken, 666
[John 15:26] Jesus says this because it accords with his general practice of referring everything he has to the one from whom he has. Compare the place where he says, "My teaching is not mine but his that sent me." If the teaching that he says was his Father's and not his own was actually his as well, how much more does the Holy Spirit proceed from him, just as he proceeds from the Father. The one from whom the Son receives his Godhead is the one from whom he can claim that the Holy Spirit proceeds.Tractatus in Iohannis Evangelium 99.7
Is receiving from the Son the same thing as proceeding from the Father? If we have to maintain that there is a difference between these two things, then at least we must admit that receiving from the Son is the same thing as receiving from the Father. The Lord declares: "He will take what is mine and declare it to you. All that the Father has is mine; therefore... he will take what is mine and declare it to you." Whatever the Spirit receives, whether it is power, strength, or doctrine, the Son says will be taken from him. He lets it be understood that it will also be taken from the Father, because whatever the Father has is his.De Trinitate 8.20
John of Damascus still spoke of the Father as the source of the Godhead, and represents the Spirit as proceeding from the Father through the Logos. This is still a relic of Greek subordinationism. The East never adopted the 'filioque' of the Synod of Toledo. It was the rock of which the East and the West split.The History of Christian Doctrines (92)
The Holy Spirit is to be believed to be of the Father and of the Son. We say that this Holy Spirit is coequal to the Father and the Son proceeds from the Father and the Son. Believe this, lest evil talk corrupt good customs.
As to the objection which they make from the Gospel (John 15:26), we respond as follows: Although in that text Truth says that the Holy Spirit proceeds from the Father, he does not add alone, and so he does not deny that the Spirit proceeds also from himself. But he names the Father alone, "because he usually refers to the Father even that which is his own," because he has it from the Father.The Sentences, Book 1, D-XI, Ch1
our tendency to skip over this issue and go directly to that of the work of the Spirit is unbalanced and unhelpful - the work which the Spirit does is directly dependent on who the Spirit is.Double Procession, 423.
Nor, by the way, can we say that the Holy Spirit does not proceed from the Son as well; it is not without point that the same Spirit is called the Spirit of the Father and of the Son.The Trinity (Brooklyn: New City Press, 1991), 174
If my understanding is correct, then both the procession and the sending mentioned in the verse take place in history. Now, that understanding does not make the verse irrelevant to the doctrine of eternal procession, for as we have seen it is legitimate to find an analogy between the historical and the eternal relationships among the persons of the Trinity. But if both the procession and the sending of John 15:26 take place in time, that would support, by analogy, the view that the Spirit's eternal procession is from both Father and Son.Systematic Theology