(38) Why is Abraham said to have been eighty and six years old when Hagar bore Ishmael to him? (#Ge 16:16). Because the number which follows eighty, that is to say six, is the first perfect number, being equal to its parts, and being the first number which is composed of the multiplication of an odd and an even number; receiving also something from its efficient cause according to the odd or redundant number, and from its material and effective cause according to the even number. On which account, among the most ancient of our ancestors, some persons have called it matrimony, and others harmony; and our sacred historian too has divided the creation of the world into six days. But among numbers, eighty rejoices in perfect harmony, since it is composed of two generous diameters in a double and treble proportion, according to the figure of a square of four sides. And it contains within itself all the four inferences; the arithmetical, and the geometrical, and the harmonious one. Being in the first place composed of double numbers, as of six, eight, nine, twelve, the union of which makes thirty-five; in the second place of triple number, six, nine, twelve, eighteen, the sum of which amounts to forty-five. And from these two numbers thirty-five and forty-five, the whole number eighty is completed. Again, when the sacred historian Moses himself began by divine inspiration to utter the oracular precepts which he was commissioned to deliver, he was eighty years old. And the first man who existed of our nation according to the law of circumcision, being circumcised on the eighth day, being eminent for virtue, bears that name of joy, being called Isaac in the Chaldaic tongue, and Isaac means laughter; being naturally called so because nature rejoices or laughs at everything, being never vexed at any thing which is done in the world, but rather looking with complacency on every thing which occurs as being done well and profitably.