I answer that, Venial sin is so called from venia (pardon).  Consequently a sin may be called venial, first of all, because it has been pardoned:  thus Ambrose says that “penance makes every sin venial”:  and this is called venial “from the result.”  Secondly, a sin is called venial because it does not contain anything either partially or totally, to prevent its being pardoned:  partially, as when a sin contains something diminishing its guilt, e.g. a sin committed through weakness or ignorance:  and this is called venial “from the cause”:  totally, through not destroying the order to the last end, wherefore it deserves temporal, but not everlasting punishment.  It is of this venial sin that we wish to speak now.

For as regards the first two, it is evident that they have no determinate genus:  whereas venial sin, taken in the third sense, can have a determinate genus, so that one sin may be venial generically, and another generically mortal, according as the genus or species of an act is determined by its object.  For, when the will is directed to a thing that is in itself contrary to charity, whereby man is directed to his last end, the sin is mortal by reason of its object.  Consequently it is a mortal sin generically, whether it be contrary to the love of God, e.g. blasphemy, perjury, and the like, or against the love of one’s neighbor, e.g. murder, adultery, and such like:  wherefore such sins are mortal by reason of their genus.  Sometimes, however, the sinner’s will is directed to a thing containing a certain inordinateness, but which is not contrary to the love of God and one’s neighbor, e.g. an idle word, excessive laughter, and so forth:  and such sins are venial by reason of their genus.

Nevertheless, since moral acts derive their character of goodness and malice, not only from their objects, but also from some disposition of the agent, as stated above (Q. 18, AA. 4, 6), it happens sometimes that a sin which is venial generically by reason of its object, becomes mortal on the part of the agent, either because he fixes his last end therein, or because he directs it to something that is a mortal sin in its own genus; for example, if a man direct an idle word to the commission of adultery.  In like manner it may happen, on the part of the agent, that a sin generically mortal because venial, by reason of the act being imperfect, i.e. not deliberated by reason, which is the proper principle of an evil act, as we have said above in reference to sudden movements of unbelief.

Reply Obj. 1:  The very fact that anyone chooses something that is contrary to divine charity, proves that he prefers it to the love of God, and consequently, that he loves it more than he loves God.  Hence it belongs to the genus of some sins, which are of themselves contrary to charity, that something is loved more than God; so that they are mortal by reason of their genus.

Reply Obj. 2:  This argument considers those sins which are venial from their cause.