St. Hilary on the Biblical Canon

St. Hilary of Poitiers ca. 300-368

The reason for reckoning twenty-two books of the Old Testament is that this corresponds with the number of the [Hebrew] letters. They are counted thus according to old tradition: the books of Moses are five, Joshua son of Nun the sixth, Judges and Ruth the seventh, first and second Kings the eighth, third and fourth [Kings] the ninth, the two of Chronicles make ten, the words of the days of Ezra the eleventh, the book of Psalms twelfth, of Solomon the Proverbs, Ecclesiastes, and Song of Songs are thirteenth, fourteenth, and fifteenth, the Twelve Prophets sixteenth, then Isaiah and Jeremiah (with Lamentations and the Epistle [of Jeremiah]) and Daniel and Ezekiel and Job and Esther complete the number of the books at twenty-two. To this some add Tobit and Judith to make twenty-four books, according to the number of the Greek letters, which is the language used among Hebrews and Greeks gathered in Rome. (Tractates on the Psalms 15)