There is a method to prove what is internal and what is external. Degrees of freedom and control. I can control internal thoughts/actions greater than external actions, and some external actions are out of my control.
Hence, there is an internal world (mind/neurons/code/maths) a physical body (flesh/bone/matter) and a physical outside world (rocks, gravity, earth, sun etc).
Add to that the observation of people and of self, and you get other implications that are unavoidable.