What is a "human act" in Catholic moral theology, and how does it relate to "full consent of the will" which is one of the conditions for a mortal sin?
Prümmer, O.P., Handbook of Moral Theology pp. 7-8:
9. Three definitions which do not differ essentially from each other are usually given: a human act is one which proceeds from the deliberate will of man; or, an act which proceeds from the knowledge of the intellect and the impulse of the will; or, an act of which man himself is the master.
Therefore the following acts are not human:
Acts which proceed from the vegetative faculty in man, such as nutrition, the pulse of the heart, etc. For man is not the master of such acts, since they do not proceed from his intellect and will.
Acts elicited by those who are completely mad, by drunkards, by children without the use of their reason, by those under hypnotic influence. Such acts are not directed by a deliberate will.
The so-called "actus primo-primi" which occur so quickly that they precede the use of the reason and will, such as those sudden feelings of anger which arise from an unexpected insult. On the other hand those acts which are called "secundo-primi" and "secundo-secundi" are human acts, although the former are not so perfect as the latter through lack of complete attention.
Forced acts which are performed under the influence of external violence. Man is not the master of such acts.
Full consent & mortal sin
ibid. p. 69:
Three conditions must be verified for mortal sin:
- There must be grave matter which is determined by the object and circumstances of the act (or omission) and which is made known to us as such in the first place through the teaching authority of the Church and her theologians. There are some sins which do not admit of slight matter and these are mortal sins "ex toto genere suo" (v.g., lust, blasphemy, etc.); in other sins the matter is not always grave (v.g., in theft, or fasting), and thus the sin may be venial. These are mortal sins "ex genere suo".
- Full advertence to the moral nature of the act is required. Therefore where such advertence is defective, sins are always venial (or there may be no sin at all) because of the act's imperfection.
- Full consent is also necessary, and this is always presumed to be present where there is full advertence and no external violence. Therefore fear and passion certainly diminish consent but do not destroy it, and they do not prevent mortal sin unless full advertence is lacking.
In practice there is often reasonable doubt whether all these three conditions were verified or not in a particular case. In such instances, if the confessor fails to reach a conclusion after prudent enquiry he must leave the matter to God's judgement but warn the penitent against future dangers of sinning. If the penitent is in doubt about one or other of the requisite conditions he should candidly reveal his doubts to the confessor and submit to his judgement.
In the following instances advertence and consent are imperfect:
1. when an act is committed while half asleep or half drunk;
2. when the penitent is not in complete possession of himself, v.g., because of a sudden and most vehement surge of passion;
3. when a person is suffering from pathological states or feelings, e.g., hysteria, mania, phobia;
4. when the penitent can truthfully and certainly assert that he would never have acted in that way if he had thought about it seriously beforehand;
5. when the penitent in the face of temptation is immediately agitated and sorry because of the devout state of his conscience.
It depends how one got into the situation of "sleep[ing] with someone other then his husband or wife". If it's an indirectly willed act (e.g., one lost his ability to make informed decisions, such as by getting drunk; cf. §94 of Moral Theology), then this adultery certainly is a formal sin. If one was tricked into it (as Jacob was with Lia in Gen. 29:23-25), then it is only a material sin of adultery.