Counted and computed: the thirteen attributes of mercy (Exodus 34:6–7); 13 = echad = ahavah in hechrachi; the age of bar mitzvah (Avot 5:21).
Thirteen is the tradition's number of mercy — the thirteen attributes recited on fast days and through the New Year — and at the same time the number of unity and of love, since echad and ahavah are both 13. The two facts are unrelated, but they are constantly preached together, because a God whose name is One and whose attributes are thirteen mercies is being described as one God. The bar mitzvah at thirteen adds a third, legal, sense to the number.
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