From St Thomas Aquinas’ Inaugural Lectures:
The Old Testament is divided according to the teaching of the commandments, for the commandments is of two kinds, the binding and the warning. The binding is the command of a king who can punish transgressors. Proverbs 20:2: ‘As the roaring of a lion, so also is the dread of a king.’ But a warning is the precept of a father who must teach. Sirach 7:25: ‘Do you have you children? Instruct them.’ The precept of a king is of two kinds, one which establishes the laws, another which induces to observance of the law, which is customarily done through is heralds and ambassadors. Thus it is that three kinds of commands are distinguished, that of the king, that of the herald and that of the father. On this basis the Old Testament is subdivided into three parts according to Jerome in his prologue. to the book of Kings.
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The first part is contained in the law which is proposed by the king himself. Isaiah 33:22: ‘For the Lord is our judge, the Lord is our lawgiver, the Lord is our King.’
The second is contained in the prophets who were as it were, ambassadors and heralds of God, speaking to the people in the person of God, and urging them to observance of the law. Haggai 11:13: ‘ And Haggai, the messenger of the Lord, as one of the messengers of the Lord, spoke.’
The third is contained in the works of hagiographers, writers who were inspired by the Holy Spirit and spoke as for themselves and not for God. Hence they are called saintly writers because they were writers of the sacred, agios meaning ‘sacred’, and graphia meaning ‘scriptures’. Thus the precepts found in them are paternal. As is evident in Proverbs 6:20: ‘My son, keep the commandments of your father.’