(i) The generated thing derives existence from its generator so that which is generated is a derived existence.
(ii) A derived existence cannot be a self-subsistence.
(iii) The divine existence is self-subsisting.
(iv) Thus a generated existence cannot be the divine existence.
2. What is generated in God receives its existence from the generator but this does not negate divine self-subsistence because the generated existence is not received into a subject. Rather, the generated Son receives His existence from another but not as if He was different from the divine nature, because the perfection of the divine existence contains the Son proceeding by way of intelligible action, as well as the Father Who is the principle of the Son. For God's existence is the same as His act of understanding. A helpful analogy is that the sun's rays have existed just as long as the sun has but the rays proceed from the sun while the sun does not proceed from the rays. The Son is autotheos in the sense of being true God, lacking none of the perfections of the Father. But He is not personally autotheos but essentially autotheos; He is not autotheos relatively as Son since He is from the Father but He is autotheos absolutely because His essence is the one essence which exists of itself and is not divided or produced from another essence. This is the truth which Christ teaches when He says [Jn 5:26], "Just as the Father has life in Himself, so He has granted the Son to have life in Himself." This is why St. John Damascene, Doctor of the Church, says [On the Orthodox Faith 1:10], "All things which the Son and the Spirit severally have, They have of the Father, even being itself."
3. (i) Something that always exists needs nothing in order to exist [St. Thomas Aquinas, Disputed Questions On the Power of God, q. 3, art. 13, arg. 1].
Need does not mean a defect regarding what is needed, but simply an order of origin regarding that from which it is; thus something can always exist yet need something to exist since it receives its nature from another rather than from itself [ibid., ad 1].
4. (i) Every effect is posterior to its cause [ibid., arg. 5].
(ii) Whatever is from another is the effect of that from which it exists.
(iii) Thus since the Son is from the Father He is posterior to the Father.
(iv) Thus the Son is not eternal and is not God.
5. The Son is not an effect because He is begotten, not made, as in "created" [ibid., ad 5]. He is generated which means that His nature is the same as that of the Father Who generates Him and this is the eternal divine nature [ibid.]. Thus the Father is called the source/fount instead of the "cause" of the Son (though several Eastern Doctors called Him the "cause" strictly meaning "principle of origin"). He is the principle not of duration but of origin without priority [ST I, q. 33, art. 1, ad 3]. The principle and that which proceeds therefrom are related as simultaneous according to the order of intellect (reason) and nature inasmuch as they enter into each other's definition [ST I, q. 42, art. 3, ad 2]. The relations are the Trinity of Persons subsisting in one divine nature, "so neither on the part of nature nor on the part of relation" is any person "prior to another," even according to "the order of nature and reason" [ibid.]
6. (i) Nothing receives what it already has [DQPG q. 13, art. 3, arg. 2].
(ii) Something which always exists always has nature.
(iii) Thus something which always exists does not receive its nature from another.
(iv) Since the Son is from the Father He receives His nature from the Father.
(v) Thus the Son did not always exist.
7. The Son did not have His nature before receiving His nature but He has it when He has already received it from the Father. Since the Son receives His essence from eternity, He has His nature from eternity [ibid., ad 2].
8. (i) Whatever already exists is not brought into being by any means, including via generation [ibid., arg. 3].
(ii) Thus whatever is generated does not exist at some time.
(iii) Everything from another is generated.
(iv) Thus since the Son is generated there was a time when He was not.
(v) God is eternal, i.e. there was never a time when God was not.
(vi) Therefore the Son is not God.
9. This objection is based on a misunderstanding of the eternal generation of the Son. It would hold if the eternal generation was one occurring via motion because whatever is moving towards nature does not exist yet; the Son is not successively generated [ibid., ad 3]. But with the Son there is no difference between being generated and having been generated and so it is not necessary or even possible that the Son Who is generated did not exist at some time [ibid.]. There is no difference between being generated and having been generated with the Son because, as St. Thomas Aquinas says, "in eternity the indivisible 'now' stands ever still" [ST I, q. 42, art. 2, ad 4]. The Son is ever being born but it is preferable to say He is "ever born" since this expression better conveys the permanence of eternity and His perfection [ibid.]. The Father begets the Son by nature and His nature is perfect from eternity [ibid., corp.].
10. (i) That which has existence only from another does not exist as considered in itself [DQPG q. 13, art. 3, arg. 4].
(ii) Such a thing must have not existed at some time.
(iii) Since the Son has existence from the Father there must have been some time at which He did not exist.
11. True, something which has its nature from another is in itself a non-being if it is other than the very nature it receives from the generator [ibid., ad 4]. But the Son is the nature He receives from the Father and so in nature He cannot be called a non-being or something with the potential to not exist [ibid.].
12. Thus it is clear that the doctrine of the eternal generation of the Son suffers from no defects and preserves the full divinity of the Son and guards from the Sabellian and Tritheist heresies.