A standard
philosophical question.
How can we determine that we actually exist as full bodies in the world we are experiencing? It is impossible to prove that a person is not simply a brain that is being fed immense amounts of stimuli to simulate a real world that does not truly exist.
Or for that matter, it may be possible that we are simply artificial intelligence software in a very complex computer.
After all, if we really are flesh and meat creatures, then we're still one level away from perceiving reality. You don't really experience the world, you experience a world created in your mind from the sensory input it receives from your body. Everything you think you sense is actually a recreation that is unique to you.