There is nothing like delving into Pythagorean medieval philosopher’s cogitation on your way to university on a London bus (innit):
The number one claims an exceptional position, which we meet again in the natural philosophy of the Middle Ages. According to this, one is not a number at all; the first number is two. 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. Also associated with the number two is the idea of right and left, and remarkably enough, of favourable and unfavourable, good and bad. The “other” can have a “sinister” significance – or one feels it, at least, as something opposite and alien. Therefore, argues a medieval alchemist, God did not praise the second day of creation, because on this day (Monday, the day of the moon) the binarius, alias the devil, came into existence. Two implies a one which is different and distinct from the “numberless” One. In other words, as soon as the number two appears, a unit is produced out of the original unity, and this unit is none other than that same unity split into two and turned into a “number”. The “One” and the “Other” form an opposition, but there is no opposition between one and two, for these are simple numbers which are distinguished only by their arithmetical value and by nothing else. The “One,” however, seeks to hold to its one-and-alone existence, while the “Other” ever strives to be another opposed to the One. The One will not let go of the Other because, if it did, it would lose its character; and the Other pushes itself away from the One in order to exist at all. Thus there arises a tension of opposites between the One and the Other. But every tension of opposites culminates in a release, out of which comes the “third.” In the third, the tension is resolved and the lost unity is restored. Unity, the absolute One, cannot be numbered, it is indefinable and unknowable; only when it appears as a unit, the number one, is it knowable, for the “Other” which is required for this act of knowing is lacking in the condition of the One. Three is an unfolding of the One to a condition where it can be known – unity become recognizable; had it not been resolved into the polarity of the One and the Other, it would have remained fixed in a condition devoid of every quality. Three therefore appears as a suitable synonym for a process of development in time, and thus forms a parallel to the self-revelation of the Deity as the absolute One unfolded into Three. The relation of Threeness to Oneness can be expressed by an equilateral triangle, A=B=C, that is, by the identity of the three, threeness being contained in its entirety in each of the three angles. This intellectual idea of the equilateral triangle is a conceptual model for the logical image of the Trinity.
C.G. Jung Psychology and Western Religion, from part 1: A psychological approach to the trinity, p.15