Objection 1. It seems that observance is not a special virtue, distinct from other virtues. For virtues are distinguished by their objects. But the object of observance is not distinct from the object of piety: for Tully says (De Invent. Rhet. ii) that "it is by observance that we pay worship and honor to those who excel in some kind of dignity." But worship and honor are paid also by piety to our parents, who excel in dignity. Therefore observance is not a distinct virtue from piety.
Objection 2. Further, just as honor and worship are due to those that are in a position of dignity, so also are they due to those who excel in science and virtue. But there is no special virtue whereby we pay honor and worship to those who excel in science and virtue. Therefore observance, whereby we pay worship and honor to those who excel in dignity, is not a special virtue distinct from other virtues.
Objection 3. Further, we have many duties towards those who are in a position of dignity, the fulfilment of which is required by law, according to Romans 13:7, "Render... to all men their dues: tribute to whom tribute is due," etc. Now the fulfilment of the requirements of the law belongs to legal justice, or even to special justice. Therefore observance is not by itself a special virtue distinct from other virtues.
On the contrary, Tully (De Invent. Rhet. ii) reckons observance along with the other parts of justice, which are special virtues.
I answer that, As explained above (101, 1 and 3; 80), according to the various excellences of those persons to whom something is due, there must needs be a corresponding distinction of virtues in a descending order. Now just as a carnal father partakes of the character of principle in a particular way, which character is found in God in a universal way, so too a person who, in some way, exercises providence in one respect, partakes of the character of father in a particular way, since a father is the principle of generation, of education, of learning and of whatever pertains to the perfection of human life: while a person who is in a position of dignity is as a principle of government with regard to certain things: for instance, the governor of a state in civil matters, the commander of an army in matters of warfare, a professor in matters of learning, and so forth. Hence it is that all such persons are designated as "fathers," on account of their being charged with like cares: thus the servants of Naaman said to him (2 Kings 5:13): "Father, if the prophet had bid thee do some great thing," etc.
Therefore, just as, in a manner, religion, whereby worship is given to find piety, whereby we worship our so under piety we find observance, whereby worship and honor are paid to persons in positions of dignity.
Reply to Objection 1. As stated above (101, 3, ad 2), religion goes by the name of piety by way of supereminence, although piety properly so called is distinct from religion; and in the same way piety can be called observance by way of excellence, although observance properly speaking is distinct from piety.
Reply to Objection 2. By the very fact of being in a position of dignity a man not only excels as regards his position, but also has a certain power of governing subjects, wherefore it is fitting that he should be considered as a principle inasmuch as he is the governor of others. On the other hand, the fact that a man has perfection of science and virtue does not give him the character of a principle in relation to others, but merely a certain excellence in himself. Wherefore a special virtue is appointed for the payment of worship and honor to persons in positions of dignity. Yet, forasmuch as science, virtue and all like things render a man fit for positions of dignity, the respect which is paid to anyone on account of any excellence whatever belongs to the same virtue.
Reply to Objection 3. It belongs to special justice, properly speaking, to pay the equivalent to those to whom we owe anything. Now this cannot be done to the virtuous, and to those who make good use of their position of dignity, as neither can it be done to God, nor to our parents. Consequently these matters belong to an annexed virtue, and not to special justice, which is a principal virtue. Legal justice extends to the acts of all the virtues, as stated above (Question 58, Article 6).
Source: Summa Theologica (New Advent)