More on the Filioque

The Father and the Son are identical in everything except the mutually opposed relations of fatherhood and sonship. According to the famous dictum of Athanasius, "the same things are said of the Son as are said of the Father, except His being said to be 'Father.'" The only thing that the Father alone can do is to be Father, that is, to generate the Son. Since the Father does not act alone in spirating the Spirit, the Spirit must proceed from the Father and the Son as from a single co-principle. The Holy Spirit is distinguished from the Son by a different relation of origin.

(The Filioque: What Is at Stake? by Avery Dulles, S.J.; Concordia Theological Quarterly; vol. 59, numbers 1-2; p.36)