Objection 1. It seems that covetousness is not a sin. For covetousness [avaritia] denotes a certain greed for gold [aeris aviditas*], because, to wit, it consists in a desire for money, under which all external goods may be comprised. [The Latin for covetousness "avaritia" is derived from "aveo" to desire; but the Greek philargyria signifies literally "love of money": and it is to this that St. Thomas is alluding (cf. 2, Objection 2)]. Now it is not a sin to desire external goods: since man desires them naturally, both because they are naturally subject to man, and because by their means man's life is sustained (for which reason they are spoken of as his substance). Therefore covetousness is not a sin.
Objection 2. Further, every sin is against either God, or one's neighbor, or oneself, as stated above (I-II, 72, 4). But covetousness is not, properly speaking, a sin against God: since it is opposed neither to religion nor to the theological virtues, by which man is directed to God. Nor again is it a sin against oneself, for this pertains properly to gluttony and lust, of which the Apostle says (1 Corinthians 6:18): "He that committeth fornication sinneth against his own body." On like manner neither is it apparently a sin against one's neighbor, since a man harms no one by keeping what is his own. Therefore covetousness is not a sin.
Objection 3. Further, things that occur naturally are not sins. Now covetousness comes naturally to old age and every kind of defect, according to the Philosopher (Ethic. iv, 1). Therefore covetousness is not a sin.
On the contrary, It is written (Hebrews 13:5): "Let your manners be without covetousness, contented with such things as you have."
I answer that, In whatever things good consists in a due measure, evil must of necessity ensue through excess or deficiency of that measure. Now in all things that are for an end, the good consists in a certain measure: since whatever is directed to an end must needs be commensurate with the end, as, for instance, medicine is commensurate with health, as the Philosopher observes (Polit. i, 6). External goods come under the head of things useful for an end, as stated above (117, 3; I-II, 02, 1). Hence it must needs be that man's good in their respect consists in a certain measure, in other words, that man seeks, according to a certain measure, to have external riches, in so far as they are necessary for him to live in keeping with his condition of life. Wherefore it will be a sin for him to exceed this measure, by wishing to acquire or keep them immoderately. This is what is meant by covetousness, which is defined as "immoderate love of possessing." It is therefore evident that covetousness is a sin.
Reply to Objection 1. It is natural to man to desire external things as means to an end: wherefore this desire is devoid of sin, in so far as it is held in check by the rule taken from the nature of the end. But covetousness exceeds this rule, and therefore is a sin.
Reply to Objection 2. Covetousness may signify immoderation about external things in two ways. First, so as to regard immediately the acquisition and keeping of such things, when, to wit, a man acquires or keeps them more than is due. On this way it is a sin directly against one's neighbor, since one man cannot over-abound in external riches, without another man lacking them, for temporal goods cannot be possessed by many at the same time. Secondly, it may signify immoderation in the internal affection which a man has for riches when, for instance, a man loves them, desires them, or delights in them, immoderately. On this way by covetousness a man sins against himself, because it causes disorder in his affections, though not in his body as do the sins of the flesh.
As a consequence, however, it is a sin against God, just as all mortal sins, inasmuch as man contemns things eternal for the sake of temporal things.
Reply to Objection 3. Natural inclinations should be regulated according to reason, which is the governing power in human nature. Hence though old people seek more greedily the aid of external things, just as everyone that is in need seeks to have his need supplied, they are not excused from sin if they exceed this due measure of reason with regard to riches.
Source: Summa Theologica (New Advent)