Objection 1. It seems that meanness is not a vice. For just as vice moderates great things, so does it moderate little things: wherefore both the liberal and the magnificent do little things. But magnificence is a virtue. Therefore likewise meanness is a virtue rather than a vice.
Objection 2. Further, the Philosopher says (Ethic. iv, 2) that "careful reckoning is mean." But careful reckoning is apparently praiseworthy, since man's good is to be in accordance with reason, as Dionysius states (Div. Nom. iv, 4). Therefore meanness is not a vice.
Objection 3. Further, the Philosopher says (Ethic. iv, 2) that "a mean man is loth to spend money." But this belongs to covetousness or illiberality. Therefore meanness is not a distinct vice from the others.
On the contrary, The Philosopher (Ethic. ii) accounts meanness a special vice opposed to magnificence.
I answer that, As stated above (I-II, 01, 3; I-II, 18, 6), moral acts take their species from their end, wherefore in many cases they are denominated from that end. Accordingly a man is said to be mean [parvificus] because he intends to do something little [parvum]. Now according to the Philosopher (De Praedic. Cap. Ad aliquid.) great and little are relative terms: and when we say that a mean man intends to do something little, this must be understood in relation to the kind of work he does. This may be little or great in two ways: in one way as regards the work itself to be done, in another as regards the expense. Accordingly the magnificent man intends principally the greatness of his work, and secondarily he intends the greatness of the expense, which he does not shirk, so that he may produce a great work. Wherefore the Philosopher says (Ethic. iv, 4) that "the magnificent man with equal expenditure will produce a more magnificent result." On the other hand, the mean man intends principally to spend little, wherefore the Philosopher says (Ethic. iv, 2) that "he seeks how he may spend least." As a result of this he intends to produce a little work, that is, he does not shrink from producing a little work, so long as he spends little. Wherefore the Philosopher says that "the mean man after going to great expense forfeits the good" of the magnificent work, "for the trifle" that he is unwilling to spend. Therefore it is evident that the mean man fails to observe the proportion that reason demands between expenditure and work. Now the essence of vice is that it consists in failing to do what is in accordance with reason. Hence it is manifest that meanness is a vice.
Reply to Objection 1. Virtue moderates little things, according to the rule of reason: from which rule the mean man declines, as stated in the Article. For he is called mean, not for moderating little things, but for declining from the rule of reason in moderating great or little things: hence meanness is a vice.
Reply to Objection 2. As the Philosopher says (Rhet. ii, 5), "fear makes us take counsel": wherefore a mean man is careful in his reckonings, because he has an inordinate fear of spending his goods, even in things of the least account. Hence this is not praiseworthy, but sinful and reprehensible, because then a man does not regulate his affections according to reason, but, on the contrary, makes use of his reason in pursuance of his inordinate affections.
Reply to Objection 3. Just as the magnificent man has this in common with the liberal man, that he spends his money readily and with pleasure, so too the mean man in common with the illiberal or covetous man is loth and slow to spend. Yet they differ in this, that illiberality regards ordinary expenditure, while meanness regards great expenditure, which is a more difficult accomplishment: wherefore meanness is less sinful than illiberality. Hence the Philosopher says (Ethic. iv, 2) that "although meanness and its contrary vice are sinful, they do not bring shame on a man, since neither do they harm one's neighbor, nor are they very disgraceful."
Source: Summa Theologica (New Advent)