Objection 1. It would seem that the ten precepts of the decalogue are not set in proper order. Because love of one's neighbor is seemingly previous to love of God, since our neighbor is better known to us than God is; according to 1 John 4:20: "He that loveth not his brother, whom he seeth, how can he love God, Whom he seeth not?" But the first three precepts belong to the love of God, while the other seven pertain to the love of our neighbor. Therefore the precepts of the decalogue are not set in proper order.
Objection 2. Further, the acts of virtue are prescribed by the affirmative precepts, and acts of vice are forbidden by the negative precepts. But according to Boethius in his commentary on the Categories [Lib. iv, cap. De Oppos.], vices should be uprooted before virtues are sown. Therefore among the precepts concerning our neighbor, the negative precepts should have preceded the affirmative.
Objection 3. Further, the precepts of the Law are about men's actions. But actions of thought precede actions of word or outward deed. Therefore the precepts about not coveting, which regard our thoughts, are unsuitably placed last in order.
On the contrary, The Apostle says (Romans 13:1): "The things that are of God, are well ordered" [Vulgate: 'Those that are, are ordained of God']. But the precepts of the decalogue were given immediately by God, as stated above (Article 3). Therefore they are arranged in becoming order.
I answer that, As stated above (3,5, ad 1), the precepts of the decalogue are such as the mind of man is ready to grasp at once. Now it is evident that a thing is so much the more easily grasped by the reason, as its contrary is more grievous and repugnant to reason. Moreover, it is clear, since the order of reason begins with the end, that, for a man to be inordinately disposed towards his end, is supremely contrary to reason. Now the end of human life and society is God. Consequently it was necessary for the precepts of the decalogue, first of all, to direct man to God; since the contrary to this is most grievous. Thus also, in an army, which is ordained to the commander as to its end, it is requisite first that the soldier should be subject to the commander, and the opposite of this is most grievous; and secondly it is requisite that he should be in coordination with the other soldiers.
Now among those things whereby we are ordained to God, the first is that man should be subjected to Him faithfully, by having nothing in common with His enemies. The second is that he should show Him reverence: the third that he should offer Him service. Thus, in an army, it is a greater sin for a soldier to act treacherously and make a compact with the foe, than to be insolent to his commander: and this last is more grievous than if he be found wanting in some point of service to him.
As to the precepts that direct man in his behavior towards his neighbor, it is evident that it is more repugnant to reason, and a more grievous sin, if man does not observe the due order as to those persons to whom he is most indebted. Consequently, among those precepts that direct man in his relations to his neighbor, the first place is given to that one which regards his parents. Among the other precepts we again find the order to be according to the gravity of sin. For it is more grave and more repugnant to reason, to sin by deed than by word; and by word than by thought. And among sins of deed, murder which destroys life in one already living is more grievous than adultery, which imperils the life of the unborn child; and adultery is more grave than theft, which regards external goods.
Reply to Objection 1. Although our neighbor is better known than God by the way of the senses, nevertheless the love of God is the reason for the love of our neighbor, as shall be declared later on (II-II, 25, 1; II-II, 26, 2). Hence the precepts ordaining man to God demanded precedence of the others.
Reply to Objection 2. Just as God is the universal principle of being in respect of all things, so is a father a principle of being in respect of his son. Therefore the precept regarding parents was fittingly placed after the precepts regarding God. This argument holds in respect of affirmative and negative precepts about the same kind of deed: although even then it is not altogether cogent. For although in the order of execution, vices should be uprooted before virtues are sown, according to Psalm 33:15: "Turn away from evil, and do good," and Isaiah 1:16-17: "Cease to do perversely; learn to do well"; yet, in the order of knowledge, virtue precedes vice, because "the crooked line is known by the straight" (De Anima i): and "by the law is the knowledge of sin" (Romans 3:20). Wherefore the affirmation precept demanded the first place. However, this is not the reason for the order, but that which is given above. Because in the precepts regarding God, which belongs to the first table, an affirmative precept is placed last, since its transgression implies a less grievous sin.
Reply to Objection 3. Although sin of thought stands first in the order of execution, yet its prohibition holds a later position in the order of reason.
Source: Summa Theologica (New Advent)