Objection 1. It would seem that the vice of curiosity is not about sensitive knowledge. For just as some things are known by the sense of sight, so too are some things known by the senses of touch and taste. Now the vice concerned about objects of touch and taste is not curiosity but lust or gluttony. Therefore seemingly neither is the vice of curiosity about things known by the sight.
Objection 2. Further, curiosity would seem to refer to watching games; wherefore Augustine says (Confess. vi, 8) that when "a fall occurred in the fight, a mighty cry of the whole people struck him strongly, and overcome by curiosity Alypius opened his eyes." But it does not seem to be sinful to watch games, because it gives pleasure on account of the representation, wherein man takes a natural delight, as the Philosopher states (Poet. vi). Therefore the vice of curiosity is not about the knowledge of sensible objects.
Objection 3. Further, it would seem to pertain to curiosity to inquire into our neighbor's actions, as Bede observes [Comment. in 1 John 2:16]. Now, seemingly, it is not a sin to inquire into the actions of others, because according to Sirach 17:12, God "gave to every one of them commandment concerning his neighbor." Therefore the vice of curiosity does not regard the knowledge of such like particular sensible objects.
On the contrary, Augustine says (De Vera Relig. 38) that "concupiscence of the eyes makes men curious." Now according to Bede (Comment. in 1 John 2:16) "concupiscence of the eyes refers not only to the learning of magic arts, but also to sight-seeing, and to the discovery and dispraise of our neighbor's faults," and all these are particular objects of sense. Therefore since concupiscence of the eves is a sin, even as concupiscence of the flesh and pride of life, which are members of the same division (1 John 2:16), it seems that the vice of curiosity is about the knowledge of sensible things.
I answer that, The knowledge of sensible things is directed to two things. For in the first place, both in man and in other animals, it is directed to the upkeep of the body, because by knowledge of this kind, man and other animals avoid what is harmful to them, and seek those things that are necessary for the body's sustenance. On the second place, it is directed in a manner special to man, to intellective knowledge, whether speculative or practical. Accordingly to employ study for the purpose of knowing sensible things may be sinful in two ways. First, when the sensitive knowledge is not directed to something useful, but turns man away from some useful consideration. Hence Augustine says (Confess. x, 35), "I go no more to see a dog coursing a hare in the circus; but in the open country, if I happen to be passing, that coursing haply will distract me from some weighty thought, and draw me after it... and unless Thou, having made me see my weakness, didst speedily admonish me, I become foolishly dull." Secondly, when the knowledge of sensible things is directed to something harmful, as looking on a woman is directed to lust: even so the busy inquiry into other people's actions is directed to detraction. on the other hand, if one be ordinately intent on the knowledge of sensible things by reason of the necessity of sustaining nature, or for the sake of the study of intelligible truth, this studiousness about the knowledge of sensible things is virtuous.
Reply to Objection 1. Lust and gluttony are about pleasures arising from the use of objects of touch, whereas curiosity is about pleasures arising from the knowledge acquired through all the senses. According to Augustine (Confess. x, 35) "it is called concupiscence of the eyes" because "the sight is the sense chiefly used for obtaining knowledge, so that all sensible things are said to be seen," and as he says further on: "By this it may more evidently be discerned wherein pleasure and wherein curiosity is the object of the senses; for pleasure seeketh objects beautiful, melodious, fragrant, savory, soft; but curiosity, for trial's sake, seeketh even the contraries of these, not for the sake of suffering annoyance, but out of the lust of experiment and knowledge."
Reply to Objection 2. Sight-seeing becomes sinful, when it renders a man prone to the vices of lust and cruelty on account of things he sees represented. Hence Chrysostom says [Hom. vi in Matth.] that such sights make men adulterers and shameless.
Reply to Objection 3. One may watch other people's actions or inquire into them, with a good intent, either for one's own good--that is in order to be encouraged to better deeds by the deeds of our neighbor--or for our neighbor's good--that is in order to correct him, if he do anything wrong, according to the rule of charity and the duty of one's position. This is praiseworthy, according to Hebrews 10:24, "Consider one another to provoke unto charity and to good works." But to observe our neighbor's faults with the intention of looking down upon them, or of detracting them, or even with no further purpose than that of disturbing them, is sinful: hence it is written (Proverbs 24:15), "Lie not in wait, nor seek after wickedness in the house of the just, nor spoil his rest."
Source: Summa Theologica (New Advent)