If the same state of brain can be produced by other than the usual causes, you may have a visual sensation not connected in the usual way with a physical object. This sort of thing is not specially concerned with the sense of sight. It is illustrated by the familiar example of the man who feels pain in his great toe although his leg has been amputated. Such arguments make it clear that what we directly experience cannot be the external object with which physics deals, and yet it is only what we directly experience that gives us reason to believe in the world of physics.<
Source: My Philosophical Development, Chapter 9 The External World
More info.: https://russell-j.com/beginner/BR_MPD_09-030.HTM