the [eternal] generation of the Son: It is that eternal and necessary act of the first person in the Trinity, whereby He, within the divine Being, is the ground of a second personal subsistence like His own, and puts this second person in possession of the whole divine essence, without any division, alienation, or change.
While he was the first to explain the relation of the Father to the Son by employing the idea of eternal generation, he defined this so as to involve the subordination of the Second Person to the First in respect to essence... this... afforded a stepping-stone for Arius.
The Sentences, Book 1, D-IX, 8.Eternal Generation0
In God knowledge and will are one and the same, as also are foreknowledge and predestination. Thus, although the nature and will of God are one, yet it is said that the Father generated the Son by nature, not by will, and that he is God by nature, not by will.