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.

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Though outdated medically, the four types—Sanguine, Choleric, Melancholic, and Phlegmatic—remain influential in personality theory.

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.

Soul increasingly reduced to observable mental or bodily processes.

Locke: denies innate ideas—soul as tabula rasa, a passive receiver.  Start of the NATURE / NURTURE debate.

Hume: rejects a persistent self; mind is a bundle of perceptions.

La Mettrie: human soul is an effect of bodily mechanisms (*L'homme machine*).

Diderot and French materialists view soul as an illusion of matter in motion.

Mind-body dualism increasingly replaced by monist materialism.