More from Herve:
477. It matters little whether the minister also acts seriously in those accompanying acts from which his will to act as a minister of Christ can be inferred. For the circuмstances themselves: 1. cannot make a rite in itself merely natural into a rite of the kind which Christ instituted; 2. cannot make a priest really act as a minister of Christ, if internally he does not wish to do so. It remains therefore that an internal intention is required in the minister.
488. Objection 1º: The sacraments produce grace "ex opere operato" (by the deed having been done). Ergo, whenever the external rite is seriously performed, grace necessarily follows, regardless of whether the minister has a contrary internal intention, just as seed sown in the earth yields fruit and as fire burns a rope, regardless of what the farmer or the one setting the fire internally wish.
R. 1. The sacraments are worked "ex opere operato" whenever they are and are performed according to the institution of Christ; but that they may be and may formally be performed according to the institution of Christ, they depend on the internal intention of the minister confecting and administering them.[1] -- 2. Therefore the comparison with the causes cited is not valid, for these causes possess in themselves the power of acting, and immediately produce their effect and are applied, independently of any intention. If this comparison were valid, the external rite, even when accomplished merely to mimic the sacrament, would in fact be a sacrament, which is the heresy of Luther.[2]
[1]. 3, q. 64, a. 8, ad 1. [2]. Cf. Franzelin, th. 17; Billuart, diss. 5, a. 7, prob. 6º.
479. Objection 2º: It is necessary that one can be certain of the validity of the sacraments: for otherwise the salvation of the faithful, and indeed perhaps the ecclesiastical hierarchy itself, are imperiled. But in fact, unless an external intention suffices, this certitude concerning the validity of the sacraments cannot be had, for an internal intention is known only to God. Therefore an internal intention is not required.
R. Concerning the validity of the sacraments one can have moral certitude, which suffices for acting prudently, and for dispelling anxieties of spirit. Thus Leo XIII: "When someone seriously and according to the ritual adheres to the due matter and form for confecting and conferring a sacrament, from this fact [considered according to the common manner in which men act] it may be inferred that he undoubtedly intends (with an internal intention) to do what the Church does."[1] For indeed, if there be any such, they are extremely rarely found, who have such malice that while they perform the sacrament with serious exterior, they internally withhold the intention; and in such a case, the truth of the opinion of Catharinus would profit little, since a minister as perverse as this could most likely secretly falsify the matter and form of the sacrament.
But in fact Christ provided thus far for the hierarchy, promising the perpetual assistance of the Holy Spirit, lest the Church ever fail.
IN PRACTICE: Whatever one thinks in theory about the opinion of Catharinus, it is wholly illicit to follow it, since where the validity of the sacraments is concerned, the safer portion must always be chosen.[2].
[1]. Ep. _Apostolicae_curae_, 13 Sept. 1896; cf. 3, q.64, a.8, ad 2. [2]. D. 1151