I answer that, As stated above (FS, Q[91], A[3]; FS, Q[94], A[2]) when we were treating of the commandments, the precepts are to the Law what propositions are to speculative sciences, for in these latter, the conclusions are virtually contained in the first principles. Hence whoever knows the principles as to their entire virtual extent has no need to have the conclusions put separately before him. Since, however, some who know the principles are unable to consider all that is virtually contained therein, it is necessary, for their sake, that scientific conclusions should be traced to their principles. Now in practical matters wherein the precepts of the Law direct us, the end has the character of principle, as stated above (Q[23], A[7], ad 2; Q[26], A[1], ad 1): and the love of God is the end to which the love of our neighbor is directed. Therefore it behooved us to receive precepts not only of the love of God but also of the love of our neighbor, on account of those who are less intelligent, who do not easily understand that one of these precepts is included in the other.
Reply to Objection 1: Although charity is one virtue, yet it has two acts, one of which is directed to the other as to its end. Now precepts are given about acts of virtue, and so there had to be several precepts of charity.
Reply to Objection 2: God is loved in our neighbor, as the end is loved in that which is directed to the end; and yet there was need for an explicit precept about both, for the reason given above.
Reply to Objection 3: The means derive their goodness from their relation to the end, and accordingly aversion from the means derives its malice from the same source and from no other
Reply to Objection 4: Love of our neighbor includes love of God, as the end is included in the means, and vice versa: and yet it behooved each precept to be given explicitly, for the reason given above.
Whether two precepts of charity suffice?
Objection 1: It would seem that two precepts of charity do not suffice. For precepts are given about acts of virtue. Now acts are distinguished by their objects. Since, then, man is bound to love four things out of charity, namely, God, himself, his neighbor and his own body, as shown above (Q[25], A[12]; Q[26]), it seems that there ought to be four precepts of charity, so that two are not sufficient.
Objection 2: Further, love is not the only act of charity, but also joy, peace and beneficence. But precepts should be given about the acts of the virtues. Therefore two precepts of charity do not suffice.
Objection 3: Further, virtue consists not only in doing good but also in avoiding evil. Now we are led by the positive precepts to do good, and by the negative precepts to avoid evil. Therefore there ought to have been not only positive, but also negative precepts about charity; and so two precepts of charity are not sufficient.
On the contrary, Our Lord said ( Mat. 22:40 ): "On these two commandments dependeth the whole Law and the prophets."
I answer that, Charity, as stated above (Q[23], A[1]), is a kind of friendship. Now friendship is between one person and another, wherefore Gregory says (Hom. in Ev. xvii): "Charity is not possible between less than two": and it has been explained how one may love oneself out of charity (Q[25], A[4]). Now since good is the object of dilection and love, and since good is either an end or a means, it is fitting that there should be two precepts of charity, one whereby we are induced to love God as our end, and another whereby we are led to love our neighbor for God's sake, as for the sake of our end
Reply to Objection 1: As Augustine says (De Doctr. Christ. i, 23), "though four things are to be loved out of charity, there was no need of a precept as regards the second and fourth," i.e. love of oneself and of one's own body. "For however much a man may stray from the truth, the love of himself and of his own body always remains in him." And yet the mode of this love had to be prescribed to man, namely, that he should love himself and his own body in an ordinate manner, and this is done by his loving God and his neighbor.
Reply to Objection 2: As stated above (Q[28], A[4]; Q[29], A[3]), the other acts of charity result from the act of love as effects from their cause. Hence the precepts of love virtually include the precepts about the other acts. And yet we find that, for the sake of the laggards, special precepts were given about each act---about joy ( Phil. 4:4 ): "Rejoice in the Lord always"---about peace ( Heb. 12:14 ): "Follow peace with all men"---about beneficence ( Gal. 6:10 ): "Whilst we have time, let us work good to all men"---and Holy Writ contains precepts about each of the parts of beneficence, as may be seen by anyone who considers the matter carefully.
Reply to Objection 3: To do good is more than to avoid evil, and therefore the positive precepts virtually include the negative precepts. Nevertheless we find explicit precepts against the vices contrary to charity: for, against hatred it is written ( Lev. 12:17 ): "Thou shalt not hate thy brother in thy heart"; against sloth ( Ecclus. 6:26 ): "Be not grieved with her bands"; against envy ( Gal. 5:26 ): "Let us not be made desirous of vainglory, provoking one another, envying one another"; against discord ( 1 Cor. 1:10 ): "That you all speak the same thing, and that there be no schisms among you"; and against scandal ( Rom. 14:13 ): "That you put not a stumbling-block or a scandal in your brother's way."
Whether it is fittingly commanded that man should love God with his whole heart?
Objection 1: It would seem that it is unfittingly commanded that man should love God with his whole heart. For the mode of a virtuous act is not a matter of precept, as shown above (A[1], ad 1; FS, Q[100], A[9]). Now the words "with thy whole heart" signify the mode of the love of God. Therefore it is unfittingly commanded that man should love God with his whole heart.
Objection 2: Further, "A thing is whole and perfect when it lacks nothing" (Phys. iii, 6). If therefore it is a matter of precept that God be loved with the whole heart, whoever does something not pertaining to the love of God, acts counter to the precept, and consequently sins mortally. Now a venial sin does not pertain to the love of God. Therefore a venial sin is a mortal sin, which is absurd.
Objection 3: Further, to love God with one's whole heart belongs to perfection, since according to the Philosopher (Phys. iii, text. 64), "to be whole is to be perfect." But that which belongs to perfection is not a matter of precept, but a matter of counsel. Therefore we ought not to be commanded to love God with our whole heart.
On the contrary, It is written ( Dt. 6:5 ): "Thou shalt love the Lord thy God with thy whole heart."
I answer that, Since precepts are given about acts of virtue, an act is a matter of precept according as it is an act of virtue. Now it is requisite for an act of virtue that not only should it fall on its own matter, but also that it should be endued with its due circumstances, whereby it is adapted to that matter. But God is to be loved as the last end, to which all things are to be referred. Therefore some kind of totality was to be indicated in connection with the precept of the love of God.
Reply to Objection 1: The commandment that prescribes an act of virtue does not prescribe the mode which that virtue derives from another and higher virtue, but it does prescribe the mode which belongs to its own proper virtue, and this mode is signified in the words "with thy whole heart."
Reply to Objection 2: To love God with one's whole heart has a twofold signification. First, actually, so that a man's whole heart be always actually directed to God: this is the perfection of heaven. Secondly, in the sense that a man's whole heart be habitually directed to God, so that it consent to nothing contrary to the love of God, and this is the perfection of the way. Venial sin is not contrary to this latter perfection, because it does not destroy the habit of charity, since it does not tend to a contrary object, but merely hinders the use of charity.
Reply to Objection 3: That perfection of charity to which the counsels are directed, is between the two perfections mentioned in the preceding reply: and it consists in man renouncing, as much as possible, temporal things, even such as are lawful, because they occupy the mind and hinder the actual movement of the heart towards God.
Whether to the words, "Thou shalt love the Lord thy God with thy whole heart," it was fitting to add "and with thy whole soul, and with thy whole strength"?
Objection 1: It would seem that it was unfitting to the words, "Thou shalt love the Lord thy God, with thy whole heart," to add, "and with thy whole soul, and with thy whole strength" ( Dt. 6:5 ). For heart does not mean here a part of the body, since to love God is not a bodily action: and therefore heart is to be taken here in a spiritual sense. Now the heart understood spiritually is either the soul itself or part of the soul. Therefore it is superfluous to mention both heart and soul.