Can you trust your eyes?

I found this optical illusion on Edward Tufte's web site:


(Click on image to enlarge.)

The squares marked A and B are the same shade of gray. Hard to believe.

Matter of fact, I didn't believe it; so I tested it. I pasted the picture into a graphics program and cut a piece from one square and layed it on the other. Indeed, they are the same color. But even having just done that, when I look at the original picture I still don't believe it.

Tufte explains,

Color is intensely contextual, like design elements in general.

Marvin Gaye explains,

People say believe half of what you see,
Son, and none of what you hear
But I can't help bein' confused
If it's true please tell me dear