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What role did atoms play in Democritus’s theory of the human soul?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Atomism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

In Democritus’s atomic theory, the soul is not an immaterial essence but is composed of physical atoms just like the rest of the body.1

Democritus posited that the soul consists of specific, highly mobile atoms that allow it to function as a mechanical part of the human system. His theory regarding the soul’s role includes the following key points:

Interaction through Contact: The atoms of the soul are affected only by the physical contact of other atoms. Sensations are understood as changes produced in the soul when it is impinged upon by atoms emitted from external objects.1
Mechanical Perception: Because the soul is material, perception is a physical process. For example, the sensation of taste is determined by the size and shape of the atoms contacting the soul; round atoms are associated with sweet tastes, while rough or uneven atoms produce different sensory experiences.1
Tranquility: In his ethical philosophy, Democritus defined the ultimate good as a state of "cheerfulness," which he described as a condition where the soul exists in peace and tranquility, free from the disturbances of fear or superstition.1