The four temperaments originate in Ancient Greek medicine.

Hippocrates (5th century BCE) proposed that health and personality are governed by four bodily fluids or “humors.”

Later, Galen (2nd century CE) systematized this theory and linked the humors to psychological traits.

Each temperament was thought to result from an excess of one humor: blood, yellow bile, black bile, or phlegm.

Though outdated medically, the four types—Sanguine, Choleric, Melancholic, and Phlegmatic—remain influential in personality theory.

Franz Joseph Gall proposed that mental faculties were localized in the brain.

Bumps on the skull supposedly indicated personality traits.

Phrenology gained popular appeal in 19th-century Europe and America.

Although flawed, it encouraged anatomical and neurological research.

Foreshadowed modern brain imaging and cognitive neuroscience.

Gustav Fechner studied the quantitative relationship between stimulus and sensation.

"psycho-physics is an exact doctrine of the relation of function or dependence between body and soul" (Elemente der Psychophysik, 1860)

His law: sensation increases logarithmically with stimulus intensity.

Considered one of the founders of experimental psychology.

Merged philosophy and empirical science through measurement.

Inspired later studies of perception and thresholds.

Renewed interest in Platonism and Hermetic soul cosmologies.

Descartes: radical dualism—soul (res cogitans) and body (res extensa).

Human soul seen as seat of reason, will, and self-consciousness.

Debates emerge over animal souls and mechanistic bodies.

Mystical and esoteric views on soul persisted alongside rationalism.

Soul increasingly tied to the concept of personhood.