Constitutionalism is the philosophy that there should exist some limit on the authority of the sovereign, or on the power of the established government. Constitutionalism does not rely on the existence of a physical document like the United States Constitution; instead, it usually relies on tradition. One of the key constitutionalist thinkers was Juan de Mariana, who lived in Spain in the 16th and 17th centuries. He believed that kings must be held accountable by the people for their actions. He also believed that tyrants could be legitimately killed by any individual acting on his own authority. The concept of the legitimacy of tyrannicide was not new to the world at that time. Scholastic philosophers had touched on it but never came up with specific circumstances in which it was lawful. According to Mariana, violation of the laws of religion, taxation without the peoples’ consent, or prevention of the meeting of a democratic parliament were all considered to be acts of tyranny, and therefore incurred the punishment of regicide. Mariana’s work that exhibited this concept, De Rege et Regis Institutione (On the King and the Royal Institution), caused outrage in France after king Henry IV was assassinated in 1610. Interestingly, the king of Spain did not imprison or arrest Mariana for his writings on the legitimacy of regicide. However, when Mariana published his work criticizing the act of debasing the currency, De Monetae Mutatione (On the Alteration of Money), Philip III of Spain (who was debasing the currency) imprisoned him for four months. That went to show what matters were of chiefest concern to the king. Juan de Mariana was a very important figure in the development of Constitutionalism. His somewhat revolutionary ideas on the specific circumstances that legitimized regicide contributed greatly to the philosophy’s development.