Atheist asked: “What are the flaws in Aquinas’s argument?”
“We see that things which lack knowledge, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that they achieve their end, not fortuitously, but designedly. Now whatever lacks knowledge cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is directed by the archer. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.” — Thomas Aquinas
I would not refer to Aquinas’ argument for design and predestination of the soul as flawed but perhaps a misunderstanding of God the Father, His desire for relationship and thus the necessity for free will. It is true that God is immutable; that is, He does not change e.g. His divine nature is fixed (Malachi 3:6); His attributes are eternal but God is also a God consumed with love (1 John 4:8) and that love has motivated the creation of both the angelic realm and the creation of human kind.
In order for relationship with God to exist in purity, undefiled, with authenticity, created beings designed to relate to God with intimacy MUST possess free will to choose to love or hate; to choose good or evil; to choose obedience or disobedience; otherwise, love and relationship would be virtually impossible as love cannot exist in an atmosphere of coercion and authentic relationship cannot manifest void volition i.e. a sincere desire to commune with another. It is at this juncture that I disagree with Aquinas in that God does definitely move Time and Eternity toward a foreordained conclusion but the PROCESS by which that conclusion comes to fruition is FLUID; in other words, though God’s Plan for Time and Eternity is immutable and all that the prophets have exclaimed will come to pass, God, in His omnipotence, chooses to endow His created beings with free will so that an interpersonal relationship with Him can manifest with intimacy – or – those created beings can choose to reject intimacy with God while simultaneously via His omnipotence God meticulously works through and around our personal choices in this life to bring His ultimate Plan for Time and Eternity to pass in accordance with His perfect will.
For all things to be predestined by God would render the love and justice and compassion and grace of God of no effect and all life nihilistic and futile and most tragically, the suffering of Christ meaningless.