It's all done with mirrors: proof of non-existence.

@article{Kennedy1985ItsAD,
  title={It's all done with mirrors: proof of non-existence.},
  author={John M. Kennedy},
  journal={Perception},
  year={1985},
  volume={14 4},
  pages={513-4}
}
Mirrors can show us to be absent (with minimal cognitive damage) and also can show imaginary lines to be unreal (again with little or no effect on the adjoining percept). The processes that create the subjective lines are immune to the optic information that the lines do not exist.