I answer that, Certain terms do not appear to be mutually opposed, if taken in their proper sense, whereas they are opposed if taken metaphorically:  thus “to smile” is not opposed to “being dry”; but if we speak of the smiling meadows when they are decked with flowers and fresh with green hues this is opposed to drought.  In like manner if mortal be taken literally as referring to the death of the body, it does not imply opposition to venial, nor belong to the same genus.  But if mortal be taken metaphorically, as applied to sin, it is opposed to that which is venial.

For sin, being a sickness of the soul, as stated above (Q. 71, A. 1, ad 3; Q. 72, A. 5; Q. 74, A. 9, ad 2), is said to be mortal by comparison with a disease, which is said to be mortal, through causing an irreparable defect consisting in the corruption of a principle, as stated above (Q. 72, A. 5).  Now the principle of the spiritual life, which is a life in accord with virtue, is the order to the last end, as stated above (Q. 72, A. 5; Q. 87, A. 3):  and if this order be corrupted, it cannot be repaired by any intrinsic principle, but by the power of God alone, as stated above (Q. 87, A. 3), because disorders in things referred to the end, are repaired through the end, even as an error about conclusions can be repaired through the truth of the principles.  Hence the defect of order to the last end cannot be repaired through something else as a higher principle, as neither can an error about principles.  Wherefore such sins are called mortal, as being irreparable.  On the other hand, sins which imply a disorder in things referred to the end, the order to the end itself being preserved, are reparable.  These sins are called venial:  because a sin receives its acquittal (veniam) when the debt of punishment is taken away, and this ceases when the sin ceases, as explained above (Q. 87, A. 6).

Accordingly, mortal and venial are mutually opposed as reparable and irreparable:  and I say this with reference to the intrinsic principle, but not to the Divine power, which can repair all diseases, whether of the body or of the soul.  Therefore venial sin is fittingly condivided with mortal sin.

Reply Obj. 1:  The division of sin into venial and mortal is not a division of a genus into its species which have an equal share of the generic nature:  but it is the division of an analogous term into its parts, of which it is predicated, of the one first, and of the other afterwards.  Consequently the perfect notion of sin, which Augustine gives, applies to mortal sin.  On the other hand, venial sin is called a sin, in reference to an imperfect notion of sin, and in relation to mortal sin:  even as an accident is called a being, in relation to substance, in reference to the imperfect notion of being.  For it is not against the law, since he who sins venially neither does what the law forbids, nor omits what the law prescribes to be done; but he acts beside the law, through not observing the mode of reason, which the law intends.