|A 12th-century depiction of the Wheel of Fortune|
from the "Garden of Delights" book by Herrad of Landsberg
Human beings recognized long ago that luck was a dish served in one of two flavors, and that one never knew what flavor one was going to get. Life had its ups and downs, and this became represented as a circle of possibilities. This was similar to the wheel of the Zodiac, turning throughout the year and bringing with it changes in life. At some point, however, the Wheel of Fortune (in Latin, the Rota Fortunæ) began to be represented in a Ferris Wheel configuration, so that the "ups" and "downs" could be portrayed visually.
At the top of the wheel is a man at the peak of good fortune: he is portrayed as a king. The wheel turns constantly, however—Boethius points out in his Consolation of Philosophy that, should the wheel stop turning, then she is no longer Fortune: this changeability is fundamental to what she does. Therefore (in this clockwise-turning representation), on the right side you see the one who was formerly on top, sliding down; near the bottom, his crown has fallen off. All is not dire, however, for on the left you see that fortune is turning better for someone else, who is ascending and will some day be on top.
The concept existed before Boethius. An astrologer of the 2nd century BCE, Vettius Valens, refers to the Zodiac as the wheel of fortunes, and a Roman playwright of the same era, Pacuvius, puts Fortuna on a spherical rock that constantly rolls by chance. Chaucer also mentions Fortune's wheel when, in "The Monk's Tale," he recounts multiple stories of men whose fortunes went from good to bad.