(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. (a) If the acts do not impede one another and the legislator is not unwilling, several laws can be fulfilled at the same time. Example: If two Masses are being said on adjoining altars, one can hear both--the one to satisfy the Sunday obligation, the other to perform a penance received.