People have created various ways to classify personality, or temperament, and it's interesting that many of them choose four general traits as their basis.

Examples:

Keirsey's Four Temperaments uses the following:

• Rationals

• Idealists
• Artisans
• Guardians

Myers-Briggs uses the following:

• introversion/extroversion

• sensate/intuitive
• thinking/feeling
• judging/perceiving

Another system uses the following:

• choleric

• phlegmatic
• sanguine
• melancholy

Another one uses four blood types...

Why do four types seem to work so well? What does this say about the way we think about personality, or the way we think in general?