You want to paint your room yellow, so you get some samples at the paint store. when you hold the sample against your white wall, it looks different from the way it looks against the green curtain. a psychologist would attribute this to perceptual constancy. visual paradoxes. contrast effects. threshold differences.
Contrast effects is a change of perception when we compare something with something else thereby noticing the difference between them. These comparisons can be color, price, taste.
Contrast effects can make a color appear darker when placed in a lighter background or that an item appear to be cheap when place after being shown a more expensive item. Contrast effects can either be positive or negative.
A positive Contrast effect is one that makes an item better than it is while a negative Contrast effect make an item worse than it is.
the answer would be voluntary exchange -
"the act of buyers and sellers freely and willingly engaging in market transactions. moreover, transactions are made in such a way that both the buyer and the seller are better off after the exchange than before it occurred."
hope this : )