"Others were vehement that mental images are just as real as those perceived with our eyes" - This sucks as a child, where you see a gymnasium floor open up beneath you. So you run to safety, just to be punished for what was an appropriate response.

Some children don't see any differentiation between their imagination and reality, so it's a matter of paying attention to how others' behave to know what to do.

Because you can't trust that the reality that you're in is shared by the people around you.