_{A man comes to the rabbi and complains about his life: "I have almost no money, my wife is a shrew, and we live in a small apartment with seven unruly kids. It's messy, it's noisy, it's smelly, and I don't want to live."
The rabbi says, "Buy a goat."
"What? I just told you there's hardly room for nine people, and it's messy as it is!"
"Look, you came for advice, so I'm giving you advice. Buy a goat and come back in a month."
In a month the man comes back and he is even more depressed: "It's gotten worse! The filthy goat breaks everything, and it stinks and makes more noise than my wife and seven kids! What should I do?"
The rabbi says, "Sell the goat."
A few days later the man returns to the rabbi, beaming with happiness: "Life is wonderful! We enjoy every minute of it now that there's no goat - only the nine of us. The kids are well-behaved, the wife is agreeable - and we even have some money!"}

-- traditional Jewish joke

Related to: Anchoring and Adjustment

Biases are “cognitive illusions” that work on the same principle as optical illusions, and a knowledge of the latter can be profitably applied to the former. Take, for example, these two cubes (source: Lotto Lab, via Boing Boing):

The “blue” tiles on the top face of the left cube are the same color as the “yellow” tiles on the top face of the right cube; if you're skeptical you can prove it with the eyedropper tool in Photoshop (in which both shades come out a rather ugly gray).

The illusion works because visual perception is relative. Outdoor light on a sunny day can be ten thousand times greater than a fluorescently lit indoor room. As one psychology book put it: for a student reading this book outside, the black print will be objectively lighter than the white space will be for a student reading the book inside. Nevertheless, both students will perceive the white space as subjectively white and the black space as subjectively black, because the visual system returns to consciousness information about relative rather than absolute lightness. In the two cubes, the visual system takes the yellow or blue tint as a given and outputs to consciousness the colors of each pixel compared to that background.

So this optical illusion occurs when the brain judges quantities relative to their surroundings rather than based on some objective standard. What's the corresponding cognitive illusion?