464. Minute fulfillment is not always necessary; for sometimes the minor details of the fulfillment of a law are expressly prescribed, sometimes they are not.
(a) If these details are required by the law itself or by the nature of the case, the law is not satisfied if they are neglected. Example: Friday abstinence ends exactly at midnight, and hence to eat meat even one minute before midnight is to break that abstinence.
(b) If the law does not prescribe minute details, these are not required for the fulfillment of the obligation; for laws should not be unduly burdensome. Example: One who is a few minutes late for Mass does not miss Mass, if he is present for the essential parts of the Mass.
465. Partial fulfillment is required of him who cannot make complete fulfillment, only when the part is commanded for its own sake; for that which is commanded by a law is considered by the lawgiver as either an indivisible unit, or as a whole composed of parts that have singly an independent moral value and obligation.
(a) If the thing commanded is morally an indivisible unit (e.g., a pilgrimage to a shrine), he who is not able to fulfill the whole law is bound to nothing. Example: One who has made a vow to go on pilgrimage to a distant sanctuary, is not bound to go part of the way, if he is unable to make the entire journey.
(b) If the thing commanded has parts that contribute to the end of the law, he who is able to fulfill only one or more such parts is obliged according to his ability; if it is certain that he can perform even a part, he is bound to that; if it is not certain that he can perform even a part, it would seem that generally he is excused from all. Examples: A cleric who can say some but not all the Hours of his Office, is obliged to say what he can. A person who can certainly abstain, but who cannot fast, is bound during Lent to abstain.
466. Simultaneous fulfillment by one act of several obligations is lawful, if the obligations differ only materially. They are said to differ only materially, if the motive of the legislator in giving different commands about the same thing is the same in each instance; they differ formally, if the legislator has a different motive in each instance. The motive is recognized either from the express declaration of the lawgiver, or from interpretation given through authority or custom.
(a) When two commands differ only materially, it can be presumed that the legislator is not unwilling that they be fulfilled by one and the same act, unless it is clear that he wishes them to be fulfilled by distinct acts. Example: If one falls sick at Easter time and receives the Viaticum, it is not necessary for him to receive Communion again in order to make his Easter duty; for the divine law of Viaticum and the church law of Easter Communion have the same motive, and hence can be fulfilled by one and the same Communion.
(b) When two commands differ formally, it can be presumed, unless the opposite is manifest, that the legislator wishes them to be complied with by distinct acts. Example: If a confessor imposes a fast as a penance, this penance cannot be performed on a fast day; for the motive of the law of fast is general, that of the sacramental penance is particular.
467. Simultaneous fulfillment by several acts of several obligations is sometimes possible, sometimes impossible. For the acts prescribed by different laws are either capable or incapable of being done at the same time. Thus, it is possible to hear a Mass and to say a penance of some Hail Marys at the same time. But it does not seem easy for an ordinary person to give attention to four or more Masses at the same time.