Why is it that he says, "Take for me a heifer of three years old, and a goat of three years old, and a raven of three years old, and a turtle dove and a pigeon?" (Genesis 15:9). He here mentions five animals, which are offered on the sacred altar; for these are divided into classes of victims, three kinds of terrestrial animals, the ox, the goat, and the sheep; and two kinds of birds, the turtle dove and the pigeon; for the sacred writer constantly tells us that the everlasting reverence of victims derived its origin from the patriarch, who was also the origin of the race: but instead of the expression, "Bring to me," he has very admirably used the words, "Take for me;" since there is nothing especially and peculiarly belonging to the creature, but everything is the gift of and blessing bestowed by God, who is altogether willing that when any one has received anything he should offer thanks for it with all his heart.
But he orders him to take every animal at the age of three years; since three is a full and perfect number, consisting of a beginning, a middle, and an end; but still we may raise the question, why of these three animals, he takes two females, the heifer, and the she-goat, and one male, the ram; may it not be perhaps because the heifer and the she-goat are offered as an atonement for sin; but the sheep is not, as sin arises from frailty, and the female is frail?