Ralph Waldo Emerson once said: "The quality of the imagination is to flow, and not to freeze." Imagination, the ability to visualize things in our mind's eye, is a fundamental part of mental life. It is crucial not just in creating works of art, but in anticipating situations and in problem-solving. It's a fanciful way of going beyond the here and now.

Christopher Columbus had to imagine the world as round before he set out on his epic voyage of discovery. And Albert Einstein, who once remarked, "I rarely think in words at all," had to imagine a universe where everything is relative before he theorized about the relativity of space and time.

But imagination varies from person to person. Like Einstein, some of us think almost exclusively in mental images while others think in words. In fact, about three out of every ten people would probably have trouble forming a mental image in any situation.

If you've never had your imagination tested, or would like to test it again, take the following quiz. It is based on several imagination tests, including one developed at Yale University.