Objection 1. It would seem that studiousness is not a part of temperance. For a man is said to be studious by reason of his studiousness. Now all virtuous persons without exception are called studious according to the Philosopher, who frequently employs the term "studious" (spoudaios) in this sense (Ethic. ix, 4,8,9). [In the same sense Aristotle says in Ethic. iii, 2, that "every vicious person is ignorant of what he ought to do."] Therefore studiousness is a general virtue, and not a part of temperance.
Objection 2. Further, studiousness, as stated (1), pertains to knowledge. But knowledge has no connection with the moral virtues which are in the appetitive part of the soul, and pertains rather to the intellectual virtues which are in the cognitive part: wherefore solicitude is an act of prudence as stated above (Question 47, Article 09). Therefore studiousness is not a part of temperance.
Objection 3. Further, a virtue that is ascribed as part of a principal virtue resembles the latter as to mode. Now studiousness does not resemble temperance as to mode, because temperance takes its name from being a kind of restraint, wherefore it is more opposed to the vice that is in excess: whereas studiousness is denominated from being the application of the mind to something, so that it would seem to be opposed to the vice that is in default, namely, neglect of study, rather than to the vice which is in excess, namely curiosity. wherefore, on account of its resemblance to the latter, Isidore says (Etym. x) that "a studious man is one who is curious to study." Therefore studiousness is not a part of temperance.
On the contrary, Augustine says (De Morib. Eccl. 21): "We are forbidden to be curious: and this is a great gift that temperance bestows." Now curiosity is prevented by moderate studiousness. Therefore studiousness is a part of temperance.
I answer that, As stated above (141, 3,4,5), it belongs to temperance to moderate the movement of the appetite, lest it tend excessively to that which is desired naturally. Now just as in respect of his corporeal nature man naturally desires the pleasures of food and sex, so, in respect of his soul, he naturally desires to know something; thus the Philosopher observes at the beginning of his Metaphysics i, 1: "All men have a natural desire for knowledge."
The moderation of this desire pertains to the virtue of studiousness; wherefore it follows that studiousness is a potential part of temperance, as a subordinate virtue annexed to a principal virtue. Moreover, it is comprised under modesty for the reason given above (Question 160, Article 2).
Reply to Objection 1. Prudence is the complement of all the moral virtues, as stated in Ethic. vi, 13. Consequently, in so far as the knowledge of prudence pertains to all the virtues, the term "studiousness," which properly regards knowledge, is applied to all the virtues.
Reply to Objection 2. The act of a cognitive power is commanded by the appetitive power, which moves all the powers, as stated above (I-II, 09, 1). Wherefore knowledge regards a twofold good. One is connected with the act of knowledge itself; and this good pertains to the intellectual virtues, and consists in man having a true estimate about each thing. The other good pertains to the act of the appetitive power, and consists in man's appetite being directed aright in applying the cognitive power in this or that way to this or that thing. And this belongs to the virtue of seriousness. Wherefore it is reckoned among the moral virtues.
Reply to Objection 3. As the Philosopher says (Ethic. ii, 93) in order to be virtuous we must avoid those things to which we are most naturally inclined. Hence it is that, since nature inclines us. chiefly to fear dangers of death, and to seek pleasures of the flesh, fortitude is chiefly commended for a certain steadfast perseverance against such dangers, and temperance for a certain restraint from pleasures of the flesh. But as regards knowledge, man has contrary inclinations. For on the part of the soul, he is inclined to desire knowledge of things; and so it behooves him to exercise a praiseworthy restraint on this desire, lest he seek knowledge immoderately: whereas on the part of his bodily nature, man is inclined to avoid the trouble of seeking knowledge. Accordingly, as regards the first inclination studiousness is a kind of restraint, and it is in this sense that it is reckoned a part of temperance. But as to the second inclination, this virtue derives its praise from a certain keenness of interest in seeking knowledge of things; and from this it takes its name. The former is more essential to this virtue than the latter: since the desire to know directly regards knowledge, to which studiousness is directed, whereas the trouble of learning is an obstacle to knowledge, wherefore it is regarded by this virtue indirectly, as by that which removes an obstacle.
Source: Summa Theologica (New Advent)