The Oldest Personality Type System in the World
The oldest personality type system in the world was developed with a sole purpose in mind: to treat illness through truly understanding the patient (I guess they could afford to spend more than 10 minutes on patient visits back then).
The Four Temperaments can be traced back to Ancient Greek medicine and philosophy, notably in the work of Hippocrates (c.460-377/359BC – the ‘Father of Medicine’) and Plato (428-348BC).
The Four Temperaments are sanguine (enthusiastic, active, and social), choleric (short-tempered, fast, or irritable), melancholic (analytical, wise, and quiet), and phlegmatic (relaxed and peaceful).
The Four Temperaments are certainly a simplification of the complex human nature, but understanding the dimensions of emotional stability/instability and extroversion/introversion can be useful for understanding and predicting someone's behavior.
While some people nicely fit into "pure" temperament type, others can have a mixture of them. Having spent 15 years as an apprentice to my NLP teacher, I've made the transition from choleric to sanguine temperament (without ever looking back), so I have the first-hand experience that it can certainly be done. The throwbacks to my biological choleric temperament, while initially frequent under heavy stress, now happen quite rarely and are immediately served with the mental/emotional "directions for getting back to my sanguine home in record time".