(a) The intrinsic mode is that which comes from the nature of the virtue commanded. Thus, in the Fourth Commandment is included not only the substance of an act (viz., that honor be shown), but also the mode of the act (i.e., that such honor and so much honor be shown as is owed to a parent by his child). The intrinsic mode is always included in a commandment along with the substance of the act prescribed (cfr. 480 sqq.).
(b) The extrinsic mode is that which belongs to some virtue different from the one commanded. This mode is not included in a commandment. Thus, if honor be shown to parents out of love of God, the mode of love of God is extrinsic to the commandment, for the commandment is concerned with the virtue of filial piety, and the mode of the act pertains to charity, which is a virtue distinct from filial piety.
1555. The intrinsic mode of performing an act of virtue is also twofold:
(a) The essential mode is that without which an act is not virtuous. Thus, he who gives to his indigent parents according to his means and their needs fulfills the essential mode of the Fourth Commandment, for, if he gave them less than he could afford and they needed, his act would not come up to the requirements of the commandment.
(b) The ideal mode of the performance of virtue is that which adds to the virtue greater goodness and value, and which is intended by a lawgiver as the end, but not as the object of his command. Thus, he who gives to his indigent parents not only sufficiently, but also with a great willingness and cheerfulness, fulfills the Fourth Commandment with greater perfection than another who supports his parents with less alacrity.
1556. The mode of the love of God prescribed in the first and great commandment is that God be loved with the whole heart, etc. But “to love with the whole heart, etc.,” can be understood in various senses.
(a) Thus, it may be understood to mean a love that is subjectively or intensively great, as when one loves God with much fervor and affection. This mode of love is ideal, since the measure of loving God is to love Him without measure, but it is not essential. The end of the commandment is that we love God ever more and more, and perform what is required with ever greater promptitude and gladness; but the commandment does not fix any certain degree of intensity, although it would be inordinate to choose to love God less intensely than we love creatures (see 1160).
(b) “To love with the whole heart” may be understood to mean a love that is objectively or appreciatively great, as when one esteems and loves God as the Supreme Good. This mode of love is essential, and hence without it the commandment is not observed. However much one loves God, if one does not love Him as the Supreme Good, one does not love Him aright, and does not practise the virtue of charity that is commanded.
1557. Love of God from the whole heart, objectively or appreciatively understood, is either actual or habitual.
(a) Actually, one loves God with one’s whole heart when there is never any interruption or distraction to one’s love, and one is continually engaged in an act of loving God above all else. This is the ideal mode of fulfilling the commandment of love, and it is also the end to which the commandment is intended to lead. But it is only in heaven, where God will be all in all (I Cor., xv. 28), that this ideal fulfillment will take place.