The idea that our personalities can be divided into fixed types is not new—it stretches back more than 2,500 years to ancient Greece. Known as the theory of the four “humours,” it was one of the earliest attempts to explain both health and character. The belief held that the balance of four bodily fluids—blood, phlegm, yellow bile, and black bile—determined not only physical well-being but also a person’s mood, temperament, and behaviour.

This theory influenced medicine, philosophy, and culture for centuries, shaping how people understood themselves and others. By the Renaissance, it was deeply embedded in European thought. Shakespeare often referenced humoral theory in his plays, using it to explain character flaws, desires, and even marital conflicts. In The Taming of the Shrew, Katherine’s fiery, headstrong behaviour is attributed to an excess of yellow bile, or “choler,” a humour associated with irritability and stubbornness. Her husband Petruchio attempts to “cure” her by controlling her diet, forbidding spicy or “hot” foods like mustard that were believed to worsen her condition.

To modern audiences, such treatments may seem strange, but in Shakespeare’s time these ideas were widely accepted as medical fact. Doctors, scholars, and writers alike used humoral theory to explain both disease and personality. A melancholic temperament, linked to black bile, suggested pensiveness or depression. Too much blood signified a sanguine, cheerful disposition, while excess phlegm was thought to cause sluggishness or laziness.

Though eventually replaced by modern medical science, the framework of the four humours left a lasting cultural imprint. Its legacy can be seen in contemporary fascination with personality classifications, from astrological signs to modern personality tests like Myers-Briggs. The appeal remains the same: a desire to categorize human nature, to find order in individuality, and to explain why we act the way we do.

The persistence of these ideas across millennia highlights how enduring the search for identity and self-understanding truly is. What began in the writings of Hippocrates and Galen continues to echo in today’s popular psychology, proving that the quest to define personality types is as old as civilization itself.