Aquinas and Ad Intra Eternal Generation
[Aquinas] argues some of God's actions are internal (ad intra) and some external (ad extra). The eternal generation of the Son and the procession of the Spirit, he argues, must be understood as acts ad intra. They are eternal and interior acts of self-differentiation within the life of God, which produce nothing outside of God.
Kevin Giles, The Eternal Generation of the Son: Maintaining Orthodoxy in Trinitarian Theology (Downers Grove, IL: InterVarsity Press, 2012), 0