Catechism of the Catholic Church

At the outset the Eastern tradition expresses the Father's character as first origin of the Spirit. By confessing the Spirit as he "who proceeds from the Father," it affirms that he comes from the Father through the Son.The Western tradition expresses first the consubstantial communion between Father and Son, by saying that the Spirit proceeds from the Father and the Son (filioque). It says this, "legitimately and with good reason," for the eternal order of the divine persons in their consubstantial communion implies that the Father, as "the principle without principle," is the first origin of the Spirit, but also that as Father of the only Son, he is, with the Son, the single principle from which the Holy Spirit proceeds. This legitimate complementarity, provided it does not become rigid, does not affect the identity of faith in the reality of the same mystery confessed.