PERTINACY. — Insofar as it is constancy in any operation or is manifested in one who is not moved by difficulties, it is nothing other than the virtue of perseverance (v.) exercised in an eminent way; but in a specific sense, as stubbornness and obstinacy, it is conceived as a vice opposed to perseverance by excess, that is, according to St. Thomas, Sum. Theol., 2a-2ae, q. 138, a. 2, as “persevering in one’s own opinion beyond what is fitting.”
This occurs by refusing to assent to the opinion of another, even if it is better, and this in order not to appear less wise. It is also the persistence in doing evil, whereas perseverance is constancy in doing good. Generally, pertinacity arises from pride, while fickleness derives from levity, a vice contrary to perseverance by weakness.
Pertinacity in denying one or more truths revealed by God and proposed by the Church to be believed enters as an essential element into the sin and crime of heresy (v.).