The jealous are possessed by a mad devil and a dull spirit at the same time.

In the quote "The jealous are possessed by a mad devil and a dull spirit at the same time," Johann Kaspar Lavater uses vivid imagery to describe the negative psychological and emotional effects of jealousy. He suggests that jealousy is a destructive force, both irrational and draining, causing the individual to be consumed by a "mad devil"—representing the intense, often uncontrollable nature of jealousy—and a "dull spirit", reflecting how jealousy can sap a person’s energy and vitality, leaving them feeling mentally and emotionally drained.

Lavater’s comparison highlights the self-destructive qualities of jealousy. The term "mad devil" conveys the uncontrollable and obsessive nature of jealousy, which can cause a person to act irrationally or out of character, driven by fear or insecurity. On the other hand, the "dull spirit" suggests that jealousy also stifles personal growth, creativity, and joy, making the individual feel apathetic, unfulfilled, or emotionally flat. This combination creates a toxic state where the jealous person is unable to think clearly or experience inner peace.

The phrase also speaks to how jealousy distorts a person’s perception of reality. Jealousy often magnifies perceived threats or injustices, making the individual more likely to focus on envy and resentment rather than positive emotions or rational thinking. The dullness of spirit refers to the inability to fully engage in life or embrace personal happiness, as the jealous person is perpetually fixated on the actions and successes of others.

Ultimately, Lavater’s quote emphasizes the toxic and paralyzing effects of jealousy on both the mind and spirit. It suggests that, rather than leading to growth or fulfillment, jealousy traps the individual in a cycle of emotional turmoil and mental stagnation, preventing them from experiencing true contentment or joy.