or the
gravity of adultery is measured, not from its being a union of
bodies, but from being a disorder in human generation. Moreover the
reviler is not the sufficient cause of unfriendliness in another man,
but is only the occasional cause of division among those who were
united, in so far, to wit, as by declaring the evils of another, he
for his own part severs that man from the friendship of other men,
though they are not forced by his words to do so. Accordingly a
backbiter is a murderer _occasionally,_ since by his words he gives
another man an occasion for hating or despising his neighbor. For
this reason it is stated in the Epistle of Clement [*Ad Jacob. Ep. i]
that "backbiters are murderers," i.e. occasionally; because "he that
hateth his brother is a murderer" (1 John 3:15).

Reply Obj. 3: Anger seeks openly to be avenged, as the Philosopher
states (Rhet. ii, 2): wherefore backbiting which takes place in
secret, is not the daughter of anger, as reviling is, but rather of
envy, which strives by any means to lessen one's neighbor's glory.
Nor does it follow from this that backbiting is more grievous than
reviling: since a lesser vice can give rise to a greater sin, just as
anger gives birth to murder and blasphemy. For the origin of a sin
depends on its inclination to an end, i.e. on the thing to which the
sin turns, whereas the gravity of a sin depends on what it turns away
from.

Reply Obj. 4: Since "a man rejoiceth in the sentence of his mouth"
(Prov. 15:23), it follows that a backbiter more and more loves and
believes what he says, and consequently more and more hates his
neighbor, and thus his knowledge of the truth becomes less and less.
This effect however may also result from other sins pertaining to
hate of one's neighbor.
_______________________

FOURTH ARTICLE [II-II, Q. 73, Art. 4]

Whether It Is a Grave Sin for the Listener to Suffer the Backbiter?

Objection 1: It would seem that the listener who suffers a backbiter
does not sin grievously. For a man is not under greater obligations
to others than to himself. But it is praiseworthy for a man to suffer
his own backbiters: for Gregory says (Hom. ix, super Ezech): "Just as
we ought not to incite the tongue of backbiters, lest they perish, so
ought we to suffer them with equanimity when they have been incited
by their own wickedness, in order that our merit may be the greater."
Therefore a man does not sin if he does not withstand those who
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