Relativity, gravity, electromagnetism were human developed concepts .. (admittedly built on accumulated knowledge which, (somewhat inconsistently), you removed in the hypothetical in which your own mind was still present and still controlling(?)). They then form the basis of our reality. There's no need to believe reality was 'a thing' awaiting 'discovery'. What was happening there is that humans were exploring their own perceptions/observations like: noticing apples always appear to fall downwards .. and then developing explanations which makes sense to us, of that. There is nothing in that which stands independent of our own observations of your 'fabric', (which is yet another physical concept invoked from your own knowledgebase .. yet that was supposed to have been removed?).