Context matters, as everyone knows. But now we find that context can "create" visual images in a person's brain. If you believe it, you can see it.
An experiment had people record whether they "saw" a gray rectangle that wasn't there. If gray rectangles were around the experimental area, observers "saw" a gray rectangle that wasn't there.
Belief is a powerful thing, and now we know it can create illusions. Of course, belief-driven delusions are even more common.