When the unforeseen and therefore involuntary effect of an action, which could have been foreseen and evaluated concretely, follows, we are still within the realm of *casus fortuitus* (fortuitous event).
Can moral imputability or legal responsibility be spoken of here?
First, a distinction must be made between the moral order and the legal order.
In the moral order, the effect of our actions and omissions is not imputable when the agent did not in fact foresee the effect, not even vaguely; in the legal order, however, the effect is often imputable when it could (or should) have been foreseen, even if it was not foreseen in any way. But even here there exists a profound differential note depending on whether it concerns *casus fortuitus* in matters of obligations or in matters of crimes.
In matters of obligations, it is a general principle (with few exceptions) that if, due to *casus fortuitus*, the debtor has been unable to perform the owed obligation exactly, he is not bound to compensate for the damage arising from the non-performance or delay (cf. art. 1218 of the Civil Code); indeed, if *casus fortuitus* renders the performance impossible, in whole or in part, the obligation is correspondingly extinguished (arts. 1256–59). Special norms are provided for the case in which the impossibility due to *casus fortuitus* arises when the debtor is already in default (art. 1221), and for some other particular cases (cf. arts. 1780, 1784, 1805, 1806, 2037 of the Civil Code, art. 402 of the Navigation Code, etc.). It must also be noted that the debtor must compensate for damage arising from *casus fortuitus* when he has expressly assumed such an obligation (special cases: arts. 1637 and 1648 of the Civil Code), which also holds in conscience.
Similarly, if an act, though objectively illicit, was committed due to *casus fortuitus*—that is, without either dolus or culpa—the author of it has no obligation to compensate for the damage that the act itself caused to another (cf. art. 2043 of the Civil Code); but here the exceptions are more numerous, because positive law (which, if just, also obliges in conscience) often places liability on a person, independently of their fault, for damage caused by those over whom they exercise some authority, or by things over which they have disposal.
A particular example of liability despite *casus fortuitus* is found in canon law for the priest who has obligations of Masses “ex stipendio,” even if the stipend has been entirely or partially lost without his fault (can. 829), by the principle that “res perit domino.” This also holds in conscience.
It is doubtful whether one is dealing with *casus fortuitus* or rather with culpa when *casus fortuitus* itself derives from a dolose or culpable activity (*casus dolo seu culpa determinatus*); but commonly this hypothesis is equated to that of culpa, save for the exceptions expressly provided by law.
In criminal law, *casus fortuitus* also exerts its efficacy as an eliminating cause of the psychic element of the crime.
Canon law, like the current legislations (cf. Italian Penal Code, art. 45), exempts from punishment acts that are delicts committed due to *casus fortuitus* (can. 2203 § 2). The reason is simple: “nulla poena sine culpa.”
Nor does the ancient distinction between “versari in re licita vel illicita,” to which canonists and moralists long adhered, have any further reason to exist, except as a presumption of predictability left to the judge.
If *casus fortuitus* derives from a culpable action, the culpable action itself is imputable, but never the unforeseen consequence that may follow. Likewise, the unforeseen consequence that occurs in an action, even if delictual, beyond the scope of the culpa or dolus is not imputable. This holds in the external forum and also in the internal forum, if it is truly a matter of something involuntary in itself and in its cause, or even of something voluntary in its cause but without foreseeing the effect. In the internal forum, however, moral culpa can exist even if the agent, though not foreseeing the effect, desired it, or, once it occurred, approved it; although in such a case the sin remains internal (affectus non effectus), and the effect must be said in the first case to have been willed but not voluntary.
On the other hand, it may also happen that the agent fails to prove the fortuitous nature of the event in the external forum, for which he remains liable to punishment, while in the internal forum he is free from culpa.