Lilies on a lake, a nice figure and ground illusion.

I suppose what it comes down to is that it's hard not to notice the absence of something. We're very good at accommodating sights, smells, sounds that are a part of the ambient world, but the absence of sound can be deafening, the absence of an image, blinding. Like Hughes Mearns little poem (which I always read to my children with great relish)

Yesterday upon the stair

I met a man who wasn’t there

He wasn’t there again today

Oh, how I wish he’d go away