In a previous thread I asked folk in this forum the question “What is reality?” and got a whole heap of interesting replies. An early reply that proved to be the most apt was In the end it sadly turned out (to misquote Omar Khayyáám’s Rubááiyáát) that : Myself when old did eagerly frequent Doctor and Saint, and heard great Argument About it and about: but evermore Came out by the same Door as in I went. I have since concluded that I was asking the wrong question. Hence this thread. Perhaps those who know what philosophers think can comment here on what isn’t reality (whatever that is.....). To start with I list some items that seem to me to be in this category. I begin with obvious examples. First, the contents of: 1. Dreams and nightmares. 2. Myths, such as the cosmology of the Mundurucu folk in the Amazon basin 3. Stories about Harry Potter Second, human constructs which have familiar and powerful physical representations, such as: 5. Movies 6. Music 7. Money Third, everyday tools that seem to me to be part of the non-real Platonic world: 8. The anti-eponymous “real” numbers 9. Zero, infinity and negative numbers, and their arithmetic 10. Imaginary numbers and complex analysis. Lastly, sophisticated systems of mathematics and physics that can serve to describe nature, like: 11. Gibb’s vector algebra, formulated in about 1880, which quickly replaced the more clumsy mathematical descriptions of nature then in use. 12. Geometric algebra that subsumes more specialised systems, such as Clifford and Cartan’s algebras 13. General Relativity, which describes gravity better than Newton did, but leaves the nature of that "real" phenomenon, gravity, still as mysterious as ever. (If you adopt a solipsist approach and doubt the "reality" of gravity itself, try ignoring it. I advise you to start in a small way, say by ignoring the existence of the bottom step when next coming downstairs).