ectypal.net

...to finally exclude completely the idea that the Son is a creature, a work of God, Athanasius argues that the Son's eternal begetting takes place within the life of God. Nothing is produced or created outside of God. "A work," he says, "is external to the [divine] nature, but the Son is a proper offspring of the essence."103 Athanasius did not contrast internal and external divine acts using the later Latin terms ad intra and ad extra, but he certainly made this very important distinction before anyone else had even thought of the idea. The Son's begetting was for him a divine work ad intra to be contrasted with the divine works of creation and salvation ad extra.