Can custom acquire the force of law
or nullify a law?
It seems that custom (consuetudo)
can neither acquire the force of law nor nullify a law:
Objection 1: As is clear
from what was said above (q. 93, a. 3 and q. 95, a.
2), human law stems from the natural law and the divine
law. But human custom cannot alter the law of
nature or the divine law. Therefore, neither can
it alter human law.
It is not the case that a good can come from many evils.
But the one who first starts to act against the law acts
badly. Therefore, it is not the case that something good
will be produced by the multiplication of acts similar to
that bad act. Now a law is a sort of good, since it is a
rule of human acts. Therefore, a law cannot be nullified
by custom in such a way that the custom itself acquires
the force of law.
Making law is the function of public personages, whose
role it is to govern the community; hence, private persons
cannot make law. But a custom increases in strength
through the acts of private persons. Therefore, custom
cannot acquire the force of a law by which some other law
But contrary to this:
In Epistola ad Casulanum Augustine says, “The
customs of the people of God, as well as what has been
instituted by the leaders, should be embraced as law. And
like those who transgress divine laws, so too those who
show contempt for ecclesiastical customs should be
Every sort of law proceeds from the lawmaker’s reason
and will—divine and natural law from God’s rational
will, and human law from the human will as regulated
by reason. Now just as, in practical matters,
a man’s reason and will are made manifest by what he
says, so too they are made manifest by what he does.
For each man evidently chooses as good what he brings
about by his action.
is manifest that the law can be both explained and changed
by human words, insofar as those words make manifest
human reason’s interior movement and conception.
Hence, the law can also be explained and changed through
acts which, especially when they are multiplied, effect
customs; moreover, these acts can cause something that
acquires the force of law, viz., because through repeated
exterior acts the will’s interior motion and reason’s
designs are effectively clarified. For when something
is repeated many times, it seems to proceed from the
deliberate judgment of reason. Accordingly, custom
has the force of law, nullifies law, and serves to interpret
Reply to objection 1:
As has been explained, natural law and divine law proceed
from the divine will. Hence, they cannot be changed by
any custom that proceeds from the human will; instead,
they could be changed only by God’s authority. And so no
custom contrary to the divine law or natural law can
acquire the force of law. For in the Synomymes
Isidore says, “Let custom cede to authority; let law and
reason subdue a depraved custom.”
Reply to objection 2:
As was explained above (q. 96, a. 6), human laws fail
in some cases, and so it is sometimes possible to act
outside the law without the act’s being bad, viz., in
a case where the law fails. And when such cases
are multiplied because of some change in the men, then
it becomes clear through custom that the standing law
is not advantageous—in just the way that this would
likewise become clear if a law were promulgated that
was verbally contrary to the standing law.
if the reason for which the first law was advantageous
still holds, then the law conquers the habit rather
than the habit the law—unless, perhaps, the law seems
disadvantageous precisely because it is not possible
given the customs of the country, which was one of the
conditions for law (cf. q. 95, a. 3). For
it is difficult to abolish a custom of the people.
Reply to objection 3:
The people in which a custom is introduced can be in
one of two conditions:
If it is a free people that can make laws for itself,
then the consensus of the whole people to observe the
practice that a custom makes manifest counts for more
than the authority of the ruler, who does not have the
power to make law except insofar as he stands in for
the people. Hence, even if particular persons
cannot make law, the whole people nonetheless can make
On the other hand, if
the people does not have free power to make law for
itself or to nullify a law made by someone in charge,
then a custom that is widespread within such a people
acquires the force of law to the extent that it is tolerated
by those who have the role of imposing law on the people.
For by this very toleration they give the appearance
of approving of what the custom has introduced.