When someone pinches you, how do you know you feel it?
Because your brain gets impulses from your arm, telling you to feel pain.
But... what if they were fake?
When you see things, your brain is getting signals from your eyes, same with taste, touch, hearing, etc.
What if your brain is not actually getting signals from the external sources you know?
What if your brain is receiving random impulses from a different source?
(Almost like the Matrix, perhaps some random electrical field.)
That would mean nothing is "real", just the way your brain interprets it is defined as your reality.