Objection 1. It would seem that curiosity cannot be about intellective knowledge. Because, according to the Philosopher (Ethic. ii, 6), there can be no mean and extremes in things which are essentially good. Now intellective knowledge is essentially good: because man's perfection would seem to consist in his intellect being reduced from potentiality to act, and this is done by the knowledge of truth. For Dionysius says (Div. Nom. iv) that "the good of the human soul is to be in accordance with reason," whose perfection consists in knowing the truth. Therefore the vice of curiosity cannot be about intellective knowledge.
Objection 2. Further, that which makes man like to God, and which he receives from God, cannot be an evil. Now all abundance of knowledge is from God, according to Sirach 1:1, "All wisdom is from the Lord God," and Wisdom 7:17, "He hath given me the true knowledge of things that are, to know the disposition of the whole world, and the virtues of the elements," etc. Again, by knowing the truth man is likened to God, since "all things are naked and open to His eyes" (Hebrews 4:13), and "the Lord is a God of all knowledge" (1 Samuel 2:3). Therefore however abundant knowledge of truth may be, it is not evil but good. Now the desire of good is not sinful. Therefore the vice of curiosity cannot be about the intellective knowledge of truth.
Objection 3. Further, if the vice of curiosity can be about any kind of intellective knowledge, it would be chiefly about the philosophical sciences. But, seemingly, there is no sin in being intent on them: for Jerome says (Super Daniel 1:8): "Those who refused to partake of the king's meat and wine, lest they should be defiled, if they had considered the wisdom and teaching of the Babylonians to be sinful, would never have consented to learn that which was unlawful": and Augustine says (De Doctr. Christ. ii, 40) that "if the philosophers made any true statements, we must claim them for our own use, as from unjust possessors." Therefore curiosity about intellective knowledge cannot be sinful.
On the contrary, Jerome [Comment. in Ep. ad Ephes. iv, 17 says: "Is it not evident that a man who day and night wrestles with the dialectic art, the student of natural science whose gaze pierces the heavens, walks in vanity of understanding and darkness of mind?" Now vanity of understanding and darkness of mind are sinful. Therefore curiosity about intellective sciences may be sinful.
I answer that, As stated above (166, 2, ad 2) studiousness is directly, not about knowledge itself, but about the desire and study in the pursuit of knowledge. Now we must judge differently of the knowledge itself of truth, and of the desire and study in the pursuit of the knowledge of truth. For the knowledge of truth, strictly speaking, is good, but it may be evil accidentally, by reason of some result, either because one takes pride in knowing the truth, according to 1 Corinthians 8:1, "Knowledge puffeth up," or because one uses the knowledge of truth in order to sin.
On the other hand, the desire or study in pursuing the knowledge of truth may be right or wrong. First, when one tends by his study to the knowledge of truth as having evil accidentally annexed to it, for instance those who study to know the truth that they may take pride in their knowledge. Hence Augustine says (De Morib. Eccl. 21): "Some there are who forsaking virtue, and ignorant of what God is, and of the majesty of that nature which ever remains the same, imagine they are doing something great, if with surpassing curiosity and keenness they explore the whole mass of this body which we call the world. So great a pride is thus begotten, that one would think they dwelt in the very heavens about which they argue." On like manner, those who study to learn something in order to sin are engaged in a sinful study, according to the saying of Jeremiah 9:5, "They have taught their tongue to speak lies, they have labored to commit iniquity."
Secondly, there may be sin by reason of the appetite or study directed to the learning of truth being itself inordinate; and this in four ways. First, when a man is withdrawn by a less profitable study from a study that is an obligation incumbent on him; hence Jerome says [Epist. xxi ad Damas]: "We see priests forsaking the gospels and the prophets, reading stage-plays, and singing the love songs of pastoral idylls." Secondly, when a man studies to learn of one, by whom it is unlawful to be taught, as in the case of those who seek to know the future through the demons. This is superstitious curiosity, of which Augustine says (De Vera Relig. 4): "Maybe, the philosophers were debarred from the faith by their sinful curiosity in seeking knowledge from the demons."
Thirdly, when a man desires to know the truth about creatures, without referring his knowledge to its due end, namely, the knowledge of God. Hence Augustine says (De Vera Relig. 29) that "in studying creatures, we must not be moved by empty and perishable curiosity; but we should ever mount towards immortal and abiding things."
Fourthly, when a man studies to know the truth above the capacity of his own intelligence, since by so doing men easily fall into error: wherefore it is written (Sirach 3:22): "Seek not the things that are too high for thee, and search not into things above thy ability... and in many of His works be not curious," and further on (Sirach 3:26), "For... the suspicion of them hath deceived many, and hath detained their minds in vanity."
Reply to Objection 1. Man's good consists in the knowledge of truth; yet man's sovereign good consists, not in the knowledge of any truth, but in the perfect knowledge of the sovereign truth, as the Philosopher states (Ethic. x, 7,8). Hence there may be sin in the knowledge of certain truths, in so far as the desire of such knowledge is not directed in due manner to the knowledge of the sovereign truth, wherein supreme happiness consists.
Reply to Objection 2. Although this argument shows that the knowledge of truth is good in itself, this does not prevent a man from misusing the knowledge of truth for an evil purpose, or from desiring the knowledge of truth inordinately, since even the desire for good should be regulated in due manner.
Reply to Objection 3. The study of philosophy is in itself lawful and commendable, on account of the truth which the philosophers acquired through God revealing it to them, as stated in Romans 1:19. Since, however, certain philosophers misuse the truth in order to assail the faith, the Apostle says (Colossians 2:8): "Beware lest any man cheat you by philosophy and vain deceit, according to the tradition of men... and not according to Christ": and Dionysius says (Ep. vii ad Polycarp.) of certain philosophers that "they make an unholy use of divine things against that which is divine, and by divine wisdom strive to destroy the worship of God."
Source: Summa Theologica (New Advent)