It feels like our Being is one and unique and therefore provides an irreducible concept of One that we are all presumably experiencing.
We only share our object reality not our sense of Being and so while for me this seems like a singular concept I can not witness what it is for you or anyone else.
They spend some 350 pages proving 1 + 1 = 2 from very primitive principles, including the existence of numbers themselves. Turns out, numbers, even One, are non-primitive constructs.