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Class represents how income is accrued.

The working class, as the name implies, earn their money by working. The upper class make their money through investments. The middle class fall in between. They make part of their income by working and part of their income through investments.

The practical implications are that if the working class stop working, they can no longer make ends meet. The middle class have the opportunity to stop working for periods of time, but not indefinitely. The upper class can survive without ever lifting a finger.