How do you propose I tell the difference between something I imagine seeing and something that is really there? In both cases there's an observable phenomenon,

How do you propose I tell the difference between something I imagine seeing and something that is really there? In both cases there's an observable phenomenon, and it is only though collision with the imagined object that I can tell the difference. If I think I see a child in the road I'm sure as hell not going to stop and wonder if I'm just imagining it before I brake.