I once sat in on a Sangha meeting where the learned monk was whipping up a lather: ‘Not-Two; Not-Three; Not-Four’, he pounded.
This is not what ‘Not-Two’ means. It is not a swipe at all notions of plurality. Once you miss the significance of ‘Two’, you can go all the way to quadrillion. And it wouldn’t make any difference.
‘Not-Two’: [Àdvaitham] is a term that predates Shūnyam. It has lots of layers and you won’t really notice them until you slip on one. But the three most relevant can be readily listed.
The first is the assumption of the ‘Independent and Separate ‘Self’. The ‘Two’ of the ‘Subject-Object Divide’. [‘Self and World’; ‘God and Man’, and so on.]
‘Not-Two’ is not an appellation, not a name for an ‘Object’ [concept, process, state, sentiment, anything you can objectify]. And the confounding of ‘Not-Two’ as a conventional reference, a name, is pervasive in the historical literature.
‘Not-Two’ actively locks in the Subject in a verbal hog-tie. You may not not say a word [or write a Post] about it.
Secondly, the open-ended: ‘Not’.
Thirdly, our reflexive tendency to abstract in Sign and Symbol [‘Doubles that Refer’] and hence make our World amenable to Logic and Language. In particular, expressions formulated as ‘Sign’, and further extended in Logic, Language and ‘Thought’. And then cheerfully contracted or expanded until we get seriously lost.
The earliest expansion of ‘Not-Two’ in the literature is as: ‘One without a Second’. More precise and by far the most succinct definition is Yājñavalkya’s take: ‘Neither inside not outside’.
‘The One’ [Sanskrit: Ekam Sat] can be found in the history of every literate tradition. From ‘The One’ of Plotinus that was the mainstay of the hugely influential European Neo-Platonic tradition with roots in the Parmenides to that of that of the Abrahamic Faiths [which gets conceptualized and reified into a later ‘Monotheism’.]
‘Not-Two’ is a more careful construction, an intentional negation. ‘The One’ is an assertion and its selective interpretations can take off on wild spins as evidenced in the literature.
‘Not-Two’. You can carry it around in your shirt-pocket. Bounce it, baby it, bully it. It will spring back to shape.
Importantly, it’s not that you have now restored a sundered whole to its original glory in an act of ecstatic mystical awakening. There simply was no division all along. Or more precisely, the notion of integration and division itself is recognized for what it is, a very man-made modeled idea.