Objection 1. It seems that pertinacity is not opposed to perseverance. For Gregory says (Moral. xxxi) that pertinacity arises from vainglory. But vainglory is not opposed to perseverance but to magnanimity, as stated above (Question 132, Article 2). Therefore pertinacity is not opposed to perseverance.
Objection 2. Further, if it is opposed to perseverance, this is so either by excess or by deficiency. Now it is not opposed by excess: because the pertinacious also yield to certain pleasure and sorrow, since according to the Philosopher (Ethic. vii, 9) "they rejoice when they prevail, and grieve when their opinions are rejected." And if it be opposed by deficiency, it will be the same as effeminacy, which is clearly false. Therefore pertinacity is nowise opposed to perseverance.
Objection 3. Further, just as the persevering man persists in good against sorrow, so too do the continent and the temperate against pleasures, the brave against fear, and the meek against anger. But pertinacity is over-persistence in something. Therefore pertinacity is not opposed to perseverance more than to other virtues.
On the contrary, Tully says (De Invent. Rhet. ii) that pertinacity is to perseverance as superstition is to religion. But superstition is opposed to religion, as stated above (Question 92, Article 1). Therefore pertinacity is opposed to perseverance.
I answer that, As Isidore says (Etym. x) "a person is said to be pertinacious who holds on impudently, as being utterly tenacious." "Pervicacious" has the same meaning, for it signifies that a man "perseveres in his purpose until he is victorious: for the ancients called 'vicia' what we call victory." These the Philosopher (Ethic. vii, 9) calls ischyrognomones, that is "head-strong," or idiognomones, that is "self-opinionated," because they abide by their opinions more than they should; whereas the effeminate man does so less than he ought, and the persevering man, as he ought. Hence it is clear that perseverance is commended for observing the mean, while pertinacity is reproved for exceeding the mean, and effeminacy for falling short of it.
Reply to Objection 1. The reason why a man is too persistent in his own opinion, is that he wishes by this means to make a show of his own excellence: wherefore this is the result of vainglory as its cause. Now it has been stated above (127, 2, ad 1; 133, 2), that opposition of vices to virtues depends, not on their cause, but on their species.
Reply to Objection 2. The pertinacious man exceeds by persisting inordinately in something against many difficulties: yet he takes a certain pleasure in the end, just as the brave and the persevering man. Since, however, this pleasure is sinful, seeing that he desires it too much, and shuns the contrary pain, he is like the incontinent or effeminate man.
Reply to Objection 3. Although the other virtues persist against the onslaught of the passions, they are not commended for persisting in the same way as perseverance is. As to continence, its claim to praise seems to lie rather in overcoming pleasures. Hence pertinacity is directly opposed to perseverance.
Source: Summa Theologica (New Advent)