The Reality You're In, And The Reality In You
Close your left eye and stare at the X with your right eye. Now don't look away. Move your phone closer, maybe further away, until my head appears to vanish. You have just found your blind spot: the place on your retina where nerves pass through on their way to the brain. You can't see anything there.
But blind spots don't create big obvious dark missing regions in our vision because our brains fill them in after the fact with whatever color or pattern it thinks should be there. What makes its fantasm really freaky, though, is that we believe in it more than we believe in what's actually in front of us.
When people are shown something like this and their blind spot is right here, the brain fills it in. So this is what they see. If then asked which Circle contains just parallel lines, the brain does not push us to trust what's actually out there.
Most people choose the parallel lines their brains have created with a guess over the ones that are fully available to them. Seeing is believing, but sometimes just making stuff up is super believing.