Obviously, as Christian Nightmares points out, this is a jab at us atheists. We don’t believe in God simply because we cannot see Him. Let’s set aside that overly simplistic slander for the moment.
On the face of it, this is nonsense. Of course I can see my brain. Granted, I can’t bounce photons off of it the way I can with something outside of my head. But I’ve had multiple MRI scans for sinus problems, and there was a brain located behind the sinus polyps.
Or maybe they’re trying to make a different point. I am my mind. The reason I cannot see my mind is because I cannot step outside myself. That could be an interesting point for pantheists. The reason we cannot see God is because we are part of God.
I’m reminded of the of the part of Good Omens where Anathema tries, and fails, to read Adam’s aura. She missed it for a very simple reason, “It was for the same reason that people in Trafalgar Square can’t see England.”