negation has no species; for "there is neither species nor
difference of non-being," as the Philosopher states (Phys. iv, text.
67). Therefore omission and commission cannot belong to the same
species.

_On the contrary,_ Omission and commission are found in the same
species of sin. For the covetous man both takes what belongs to
others, which is a sin of commission; and gives not of his own to
whom he should give, which is a sin of omission. Therefore omission
and commission do not differ specifically.

_I answer that,_ There is a twofold difference in sins; a material
difference and a formal difference: the material difference is to be
observed in the natural species of the sinful act; while the formal
difference is gathered from their relation to one proper end, which
is also their proper object. Hence we find certain acts differing
from one another in the material specific difference, which are
nevertheless formally in the same species of sin, because they are
directed to the one same end: thus strangling, stoning, and stabbing
come under the one species of murder, although the actions themselves
differ specifically according to the natural species. Accordingly, if
we refer to the material species in sins of omission and commission,
they differ specifically, using species in a broad sense, in so far
as negation and privation may have a species. But if we refer to the
formal species of sins of omission and commission, they do not differ
specifically, because they are directed to the same end, and proceed
from the same motive. For the covetous man, in order to hoard money,
both robs, and omits to give what he ought, and in like manner, the
glutton, to satiate his appetite, both eats too much and omits the
prescribed fasts. The same applies to other sins: for in things,
negation is always founded on affirmation, which, in a manner, is its
cause. Hence in the physical order it comes under the same head, that
fire gives forth heat, and that it does not give forth cold.

Reply Obj. 1: This division in respect of commission and omission, is
not according to different formal species, but only according to
material species, as stated.

Reply Obj. 2: In God's law, the necessity for various affirmative and
negative precepts, was that men might be gradually led to virtue,
first by abstaining from evil, being induced to this by the negative
precepts, and afterwards by doing good, to which we are induced by
the affirmative p