must be by some evident effect. Therefore it seems that swearing is
altogether unlawful.

_On the contrary,_ It is written (Deut. 6:13): "Thou shalt fear the
Lord thy God . . . and shalt swear by His name."

_I answer that,_ Nothing prevents a thing being good in itself, and
yet becoming a source of evil to one who makes use thereof
unbecomingly: thus to receive the Eucharist is good, and yet he that
receives it "unworthily, eateth and drinketh judgment to himself" (1
Cor. 11:29). Accordingly in answer to the question in point it must
be stated that an oath is in itself lawful and commendable. This is
proved from its origin and from its end. From its origin, because
swearing owes its introduction to the faith whereby man believes that
God possesses unerring truth and universal knowledge and foresight of
all things: and from its end, since oaths are employed in order to
justify men, and to put an end to controversy (Heb. 6:16).

Yet an oath becomes a source of evil to him that makes evil use of
it, that is who employs it without necessity and due caution. For if
a man calls God as witness, for some trifling reason, it would
seemingly prove him to have but little reverence for God, since he
would not treat even a good man in this manner. Moreover, he is in
danger of committing perjury, because man easily offends in words,
according to James 3:2, "If any man offend not in word, the same is a
perfect man." Wherefore it is written (Ecclus. 23:9): "Let not thy
mouth be accustomed to swearing, for in it there are many falls."

Reply Obj. 1: Jerome, commenting on Matt. 5:34, says: "Observe that
our Saviour forbade us to swear, not by God, but by heaven and earth.
For it is known that the Jews have this most evil custom of swearing
by the elements." Yet this answer does not suffice, because James
adds, "nor by any other oath." Wherefore we must reply that, as
Augustine states (De Mendacio xv), "when the Apostle employs an oath
in his epistles, he shows how we are to understand the saying, 'I say
to you, not to swear at all'; lest, to wit, swearing lead us to swear
easily and from swearing easily, we contract the habit, and, from
swearing habitually, we fall into perjury. Hence we find that he
swore only when writing, because thought brings caution and avoids
hasty words."

Reply Obj. 2: According to Augustine (De Serm. Dom. in Monte i. 17):
"If you have to swear, note that the necessity arises from the
infirmity of those w