veryone is bound by the natural dictate of reason to practice
fasting as far as it is necessary for these purposes. Wherefore
fasting in general is a matter of precept of the natural law, while
the fixing of the time and manner of fasting as becoming and
profitable to the Christian people, is a matter of precept of
positive law established by ecclesiastical authority: the latter is
the Church fast, the former is the fast prescribed by nature.

Reply Obj. 1: Fasting considered in itself denotes something not
eligible but penal: yet it becomes eligible in so far as it is useful
to some end. Wherefore considered absolutely it is not binding under
precept, but it is binding under precept to each one that stands in
need of such a remedy. And since men, for the most part, need this
remedy, both because "in many things we all offend" (James 3:2), and
because "the flesh lusteth against the spirit" (Gal. 5:17), it was
fitting that the Church should appoint certain fasts to be kept by
all in common. In doing this the Church does not make a precept of a
matter of supererogation, but particularizes in detail that which is
of general obligation.

Reply Obj. 2: Those commandments which are given under the form of a
general precept, do not bind all persons in the same way, but subject
to the requirements of the end intended by the lawgiver. It will be a
mortal sin to disobey a commandment through contempt of the
lawgiver's authority, or to disobey it in such a way as to frustrate
the end intended by him: but it is not a mortal sin if one fails to
keep a commandment, when there is a reasonable motive, and especially
if the lawgiver would not insist on its observance if he were
present. Hence it is that not all, who do not keep the fasts of the
Church, sin mortally.

Reply Obj. 3: Augustine is speaking there of those things "that are
neither contained in the authorities of Holy Scripture, nor found
among the ordinances of bishops in council, nor sanctioned by the
custom of the universal Church." On the other hand, the fasts that
are of obligation are appointed by the councils of bishops and are
sanctioned by the custom of the universal Church. Nor are they
opposed to the freedom of the faithful, rather are they of use in
hindering the slavery of sin, which is opposed to spiritual freedom,
of which it is written (Gal. 5:13): "You, brethren, have been called
unto liberty; only make not liberty an occasion to the flesh."
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