OK, GfS, lock your front door and then take a run at it, and attempt to pass through it. Try that again tomorrow, and the day after, and so on for a week. Try the same experiment again next year. Ask your self, "was my experience consistent?" If "yes", what are the chances that you experienced an illusion? Take a suitable number of pain killers.

Find another person prepared to try the same experiment (best of luck with that!). Did that person pass through your front door as if it wasn't there?

You might also like to consider the question: can nothing experience an illusion?