The Arians appear to have made use of a dilemma:  If God begat with will and purpose, these preceded the begetting, and so ἦν ποτε ὅτε οὐκ ἦν, there was a time when the Son was not:  if without will and purpose, then He begat in ignorance and of necessity.  The answer is fully given by Athanasius (c. Arian. iii. 58–67, pp. 425–431 in this Series).