What did William Harvey mean by calling the heart the “sun of the microcosm”?

In his dedication to King Charles I within his seminal work, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (1628), William Harvey referred to the heart as the "sun of its microcosm" to illustrate its central, life-sustaining role within the body.12

Harvey used this analogy to draw a parallel between the biological function of the heart and the political role of the monarch. By describing the heart as the "basis of its life" and the "chief member" from which all "liveliness and strength arise," he emphasized that the heart is the vital source of activity for the entire organism. He then extended this metaphor to the state, arguing that just as the heart governs the body, the king serves as the "heart of the state," acting as the foundational source from which all power and grace flow to his kingdoms. This comparison reflects Harvey’s status as a committed royalist and his view of the heart as the essential, governing organ of the human system.2