Pythagorean doctrines included a belief that the ultimate reality of the universe consists in number. They regarded even numbers as female and odd numbers as male. The number 5 was called 'marriage' because it was the product of the first even number 2 and the first odd number 3. The reason being that the ancient Greeks considered the number 1 a unit and not a proper number, which had to express a multiplicity.
Greek alchemists symbolized totality as a circle, an image of the global unity of everything that exists and can be conceived. Their reasoning led them away from unity and back gain, to use it as basis on which to 'assess the value of things'.
Unity is therefore the point of departure, a symbol of absolute origin and absolute end (universe means 'turned towards one'). But if singularity is the origin then duality must be the end!
The singular cannot exist without the plural. One cannot exist without two. A necessary definition of one is that to have 'one of a thing' really means to have one of a number of things, at a minimum, two. To 'have one of' there must be another.
There must be plural 'one', a pair of or a few or several or more - otherwise it is just have the 'thing', it would not need to be assigned a numerical value. It would be on its own in a singular state of existence - one of. But to be 'one of' it must have 'another of' just like it.
For example; If there is only one 'pen', I have a pen; it is not necessary to assign a number value 'one of' , as if to ask the question "is there another"? Therefore to give it value of existence, numerically it only exists if there is another. One cannot exist without two.
It is a world of duality. We are a body of two limbs, two eyes and two halves of the brain, two halves of the cell, a sum of two parents. There even seem to be two Gods reigning over good and evil - but I'm in two minds about this. There cannot be one without the other. Yin and Yang, proton and neutron, up and down, go and stop, heaven and hell, backwards and forwards, light and dark.
For each force there is an equal and opposite reaction. The fabric of time itself is a slave to multiplicity. 'One' can only exist if the 'other' also does.
A final word - ENTROPY - the natural order or the universe to find balance. A balance only exists between 'two' things not 'one'.