These various moral systems come into play only when the question concerns the lawfulness of an action. If the uncertainty concerns the validity of an action which must certainly be valid, it is not enough to act on mere probability unless, indeed, this is of such a nature as to make the Church certainly supply what is needed for the validity of the act. Thus, apart from necessity, these systems do not allow one to act on mere probability when the validity of the sacraments is in question.
Additionally, they do not allow one to act on mere probability when there is question of gaining an end which is obligatory, since certain means must be employed to gain a certainly required end. Hence, when eternal salvation is at stake, these systems demand more than uncertain means as justification. Moreover, their conception of justice demands equality, and as such excludes the use of probability when the established rights of another are concerned.