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Constant happiness is the philosopher's stone of the soul.

Voltaire (2016). “Voltaire – The Philosophical Works: Treatise On Tolerance, Philosophical Dictionary, Candide, Letters on England, Plato’s Dream, Dialogues, The Study of Nature, Ancient Faith and Fable, Zadig…: From the French writer, historian and philosopher, famous for his wit, his attacks on the established Catholic Church, and his advocacy of freedom of religion and freedom of expression”, p.1086, e-artnow
Constant happiness is the philosopher's stone of the soul.