First, if you do not have a copy, I'd highly recommend The Teachings of the Holy Orthodox Church published by Dormition Skete. I've almost finished it, and must say that I've got quite a bit out of it.
A good insight that this book touches upon, has direct bearing upon the filioque clause.
Aside from the theory of Augustine of Hippo (the very uniqueness of which makes it suspect) regarding the procession of the Holy Spirit, none of the Fathers dared to say just what the procession of the Holy Spirit is, or what the "begotteness" of the Son is. In fact, they explicitely said such is beyond human knowledge, and it is presumptuous to try and state what such innerworkings within God in fact "are" - that we know of them at all, is a result of God's self revelation, not human cleverness - hence, we can only know what He has chosen to tell us, and nothing more than this. Presuming to go beyond this is the begining of all heresy - the replacement (or trespassing beyond the limites) of what God has revealed, with our own rationalizations. It is no different in regard to the teaching of the Church regarding the procession of the Holy Spirit.
St.John of Damascus writes...
For the Father alone is ungenerate, no other subsistence accounts for His existence. And the Son alonge is begotten, for He was generated of the Father's essence eternally. And only the Holy Spirit proceeds from the Father's essence, not having been generated but simply proceeding. For this is the doctrine of the Holy Scriptures. The nature of this generation and this procession is quite beyond our understanding. (An Exact Exposition of the Orthodox Faith, I,18 )
I've noticed that many times in arguments between Roman Catholics and Orthodox over the filioque issue it is often pointed out that the Fathers often seem to speak of the Holy Spirit proceeding from the Son, particularly Latin Fathers.
A key problem with this confusion (and the work I mentioned above gets into this, with plenty of Patristic quotes in support of the author's explanations) is that it is typically grounded in a very post-schism-Latin problem - a refusal (consciously or unwittingly) to differentiate between the "inner/essential" matters pertaining to God, and the "economic/energetic" things which can be said of God... or put another way the "essential Trinity" as differentiated from the "economic Trinity."
According to the Church, man only knows God by His self revelation, which is something "energetic" (sometimes called "temporal", though this can be misleading, and is only true of certain acts/manifestations of God's uncreated energies.) There can be no knowledge of His essence, whether in this life or in eternity; this is something the Papists explicitly deny, in particular in their teaching on the "Beatific Vision". Beatitude (which is the state of the righteous who sleep in Christ) for them, is the intellectual apprehension of the Divine Essence. This is untrue, and anathema as far as the Fathers (both Eastern and Western) were concerned.
This is where the mischief begins...
Because the Latins no longer recognize the difference beween the "economic" and "essential" Holy Trinity, they are already in no position to properly understand the Fathers in general, in particular their own Fathers.
While the Fathers (and the Holy Scriptures) speak of the Holy Spirit proceeding solely from the Father "essentially", it is also true that energetically He is manifested through the Son, resting upon the Son, and being sent by Him.
This is why the Fathers speak in varying ways on this subject. The Eastern Fathers were generally more precise in their language, generally making it very clear whether they were speaking of the "essential" or "economic" Trinity...or when they speak in terms of a "synthesis" of both, they will say that the Holy Spirit "proceeds from the Father, through the Son", or something like this.
OTOH, the Western Fathers generally (though not always) did not explicitly make the distinction - it is something you'll generally find more implied. Sometimes they will go back and forth between the two sense in the same statement. For example, St.Paulinus writes...
There is one God the Father, and in Him one Son, and from Him Who is the Word, the one Spirit. These three Persons are one God forever. The single nature of God is God, consisting of Son, Spirit and Father; but the Son is born of the Father and the Spirit proceeds from the Father. No nature in the created order has anything in common with or may be compared to the divine nature (Poem XIX, 140)
As is pointed out by Fr.Michael Azkoul (author of this marvelous work), St.Paulinus states that the Holy Spirit is "from the Word", yet then goes onto say He "proceeds from the Father" (not "Father and the Son".) Speaking in two different ways in the same sentence, it's clear St.Paulinus has in mind, in the previous case, the "economic Trinity", where as in the latter the "essential Trinity."
Perhaps a saying which is particularly liable to being misunderstood by someone unwilling to make the Patristic essential/energetic distinction, is to be found in the following teaching of St.Hilary of Poitiers...
The Spirit of Truth proceeds from the Father; He is sent by the Son and receives from the Son. Now, all that belongs to the Father belongs to the Son, and for that reason He who receives from Him is the Spirit of God; but at the same time, He is the Spirit of Christ. The Spirit posesses the same nature as the Son, and both are identical in naure with the Father. He is the Spirit of Him Who raised Christ from the dead; but this is no other than the Spirit of Christ. The Divine Persons of Christ and the Father can be shown to differ in some respects if also it can be shown that the Spirit which is from God the Father is not the very Spirit of Christ.(On the Trinity VIII, 26)
While St.Hilary does not say the Holy Spirit proceeds from the Son (which in context here is speaking of the "trancendent Trinity"/"essential Trinity" addressed in the Nicene Creed), he later says everything but this regarding the manifestation of the Holy Spirit from the Son. Part of the confusion is also due to that phrase "temporal manifestation of the Holy Spirit", which is not wholly correct, or at least liable to be misunderstood (since the Fathers definately understood that the Holy Spirit, even in eternity, is manifested through the Son.)
However, St.Hilary lets his thought on the matter become very clear when he sais in the same work, later on...
But I cannot describe Him whose plea for me I cannot describe. As in the revelation that Thy Only-begotten was born of Thee before times eternal, when we cease to struggle with the ambiguities of language and difficulties of thought, the one certainty remains; so I hold fast that the Spirit is from Thee and through Him, although I cannot comprehend it with my intellect... (On the Trinity XII, 56)
The close of that statement is very clear; St.Hilary makes the distinction between the Holy Spirit being "from the Father" but "through" the Son.
Hence, there is in a sense an "Orthodox filioque" if by this one means the Holy Spirit proceeding from the Father, through the Son. Many times, the Fathers, both Western and Eastern, speak in such a fashion (though admittedly the latter are typically clearer in their manner of expression on this issue). Unfortunately, this is historically not what the Papists have meant.
The beginning of the controversy of the filioque clause is manifold...
1) The insertion itself is illegitimate, being a provincial intrusion into something that was accepted by the whole Church.
2) The favour with which this clause was taken into by the (then) new Frankish lords of the now Barbarian dominated west, was without a doubt due to the favour said men (particularly Charlemagne) held the writings of Augustine of Hippo in (which were not well known, at least in their totality, in the Christian East, and never were up until relatively recently), who definately taught the heretical filioque clause - an essential procession of the Holy Spirit from both the Father and the Son, based on a theory that the Holy Spirit was the "love personified" of Father and the Son for one another - misguided both for over-riding the Creed, but also for doing precisely what the Fathers universally denied was possible (defining precisely what the nature of the "procession" or "begotteness" of the Holy Spirit and the Son are). It spread largely due to the influence of these temporal rulers, whose obession with Augustine (to the almost total exclusion of other Fathers, even Latin Fathers) explains why Augustine is regarded as the "greatest Father" by so many modern Roman Catholics; a level of esteem Augustine did not possess in the west prior to this period.
3) Later, this basically Augustinian understanding of the Holy Trinity would become a dogma of the RCC - it would be come, at the Council of Florence, a condition for the "return" of the "erring Greeks" to the Pope's communion. Florence is still numbered as an "Ecumenical Council" by the Roman Catholics, and it's doctrinal formulations are "infallible dogmas" which cannot be rescinded. This Augustinian heresy would be bolstered latter on, by a new development (already well rooted throughout the west by the time Florence came about) - a complete ignorance of the real difference between the "essence" of God and His "energies". With such an ignorance/denial, the few other Patristic works (largely Latin Fathers) which the Papists would use to try and justify this innovation, would lend themselves to an easy misreading.
In modern times, I suspect most RC "theologians" realize their "case" in favour of the filioque clause looks very bad, and that it should never have been inserted into the Creed. At the very least, they believe this is bad for "ecumenism." Hence, there's been a concerted attempt to "minimalize" this error (imho, basically obfuscate it.)
One attempt, has been to say that the Holy Spirit "proceeds from the Father and the Son as 'one principle'". Such an articulation might have been acceptable, were it not for the fact that the Latins have otherwise "solemnly defined" teachings which are blatantly heretical on this subject. For while the "one principle" teaching can be understood to speak solely of the "temporal" or "economic Trinity" (how God reveals Himself, in particular, the Person of the Holy Spirit), it is intentionally vague so as to easily allow the old school, Augustinian/Florence Papal doctrine to remain unopposed. A similar gesture has been to freely allow Uniates to use the genuine form of the Nicene Creed in their services, with the tacit (even if only on paper) understanding that they accept the theology of the heretical filioque teaching (or at least do not oppose it, which seems to be key with all things involving Papal politics.) There is even talk that the Latins themselves will drop this clause from the publically used Creed...however, this would be more obfuscation on their part, if it did not include a repudiation of the Council of Florence, and similar "dogmatic" statements made by the Papacy in the past (and it's subordinates) in favour of this error.
Seraphim