To what is our external world external?

In the phsyics movement against Einstein three pillars of classical physics were challenged. The three pillars of Einsteinian physics were: determinism, causality and predictability, Firstly Max Born, provided a definitive solution to the problem of indeterminacy in quantum mechanics with his discovery that identical experiments can have different outcomes. Physical reality is indeterminate at its core In March 1927, Heisenberg announced his Uncertainty Principle: ‘the more precise our determination of the position of a subatomic particle, the less precise will be our determination of its momentum, and vice versa.’ In September, Bohr published his Principle of Complementarity: depending on how we measure light we see it as waves or particles but never both. These two principles made up what became known as the Copenhagen Interpretation. Its emphasis on the importance of observation in the creation of scientific findings led to a rethinking of the very idea of an ‘outside world’. So we still have the conundrum today and something for you bright young students out there to discuss: ‘To what is the external world external?’ Answers on a postcard and not more than 250 words.