Two is the first number because, with it, separation and multiplication begin, which alone make counting possible. With the appearance of the number two, another appears alongside the one, a happening which is so striking that in many languages 'the other' and 'the second' are expressed by the same word.
Jung establishes Two as the originary number of differentiation, identifying it with the eruption of otherness, opposition, and the sinister into the unity of the One.
, Psychology and Religion: West and East, 1958thesis