According to the Pythagoreans, ten was the essence of all numbers. Moreover, the power of 10 resided in the tetrad, or number 4. If you began at 1 and kept adding each successive number up to four, you arrived at the number 10 ( 1 + 2 + 3 + 4 = 10). In terms of single units, it was the number 10, but in terms of potentiality, it was the number 4. As a result, the Pythagoreans used to invoke the tetrad as their most sacred oath: "By him who gave the tetrad to our generation, which holds the source and root of eternal reality."

Source Walter Burkert, Lore and Science in Ancient Pythagoreanism, translated by E L Minar Jr (Cambridge, Mass Harvard University Press, 1972)