is there two or only three?

I've been thinking about symmetry and duality in space, especially since finishing reading Robert Venturi & Denise Scott Brown's book Complexity and Contradiction and seeing all that post-modernism and strange medievalism and especially Lutyen's buildings, a lot of which dealt with dualities and disunities.

In most disciplines, two is just fine, clear and uncomplicated. Math, science, zoology, romance. But with spaces, the identification of two necessitates a third - the space between. Even when we take two to it's simplest state- two masses or two voids - it can't ever be a simple duality. Even if there is no void between two voids, there must be, at a bare minimum, a mass. Otherwise there would only be one. The same is true of two masses - without the void they are one. It takes two( mass 2 and void) to define one (mass 1).

There is an easy and clear one, and an easy and clear three.

what riddles me is if there really is any 'two', or if there is only 'one' and 'three'.