19 I will come now to the passage in which I am accused of saying that— at least according to the true Hebrew text— the words “God saw that it was good” are not inserted after the second day of the creation, as they are after the first, third, and remaining ones, and of adding immediately the following comment: “We are meant to understand that there is something not good in the number two, separating us as it does from unity, and prefiguring the marriage-tie. Just as in the account of Noah's ark all the animals that enter by twos are unclean, but those of which an uneven number is taken are clean.” In this statement a passing objection is made to what I have said concerning the second day, whether on the ground that the words mentioned really occur in the passage, although I say that they do not occur, or because, assuming them to occur, I have understood them in a sense different from that which the context evidently requires. As regards the non-occurrence of the words in question (viz., “God saw that it was good”), let them take not my evidence, but that of all the Jewish and other translators— Aquila namely, Symmachus, and Theodotion. But if the words, although occurring in the account of the other days, do not occur in the account of this, either let them give a more plausible reason than I have done for their non-occurrence, or, failing such, let them, whether they like it or not, accept the suggestion which I have made. Furthermore, if in Noah's ark all the animals that enter by twos are unclean, while those of which an uneven number is taken are clean, and if there is no dispute about the accuracy of the text, let them explain if they can why it is so written. But if they cannot explain it, then, whether they will or not, they must embrace my explanation of the matter. Either produce better fare and ask me to be your guest, or else rest content with the meal that I offer you, however poor it may be.
I must now mention the ecclesiastical writers who have dealt with this question of the odd number. They are, among the Greeks, Clement, Hippolytus, Origen, Dionysius, Eusebius, Didymus; and, among ourselves, Tertullian, Cyprian, Victorinus, Lactantius, Hilary. What Cyprian said to Fortunatus about the number seven is clear from the letter which he sent to him. Or perhaps I ought to bring forward the reasonings of Pythagoras, Archytas of Tarentum, and Publius Scipio in (Cicero's) sixth book “Concerning the Common Weal.” If my detractors will not listen to any of these I will make the grammar schools shout in their ears the words of Virgil:
Uneven numbers are the joy of God.
Source: Letters (New Advent)