Contrary to what most may think, Constitutionalism doesn’t necessarily involve a written document. The key idea of constitutionalism is limiting the power of the king. Constitutionalism is based in the fact that the authority of the government comes from some fundamental law. Constitutionalist thinkers usually said that political sovereignty resided in the people, not the king. One of the most important works of constitutionalism was the Vindiciae Contra Tyrannos, which discussed whether or not the king can be resisted. John Knox, John Locke, and St. Robert Bellarmine were all thinkers who discussed constitutionalism.
Another Constitutionalist thinker was Juan de Mariana. Mariana believed that the people could take back the political authority they have given to a ruler if he becomes a tyrant. A tyrant, according to Mariana, is someone who violated the laws of religion, imposed taxes on the people without their consent, or who prevented the meeting of a democratic parliament. Mariana believed that anyone who is a citizen under the tyrant could justly assassinate him. Mariana also wrote on economics, and condemned inflation. He got in trouble for his writings on inflation, and ended up going to prison for his writings for four months.
In direct opposition to constitutionalism is absolutism. One of the greatest philosophers of absolutism was Thomas Hobbes. Hobbes said that human nature was not rational and instead directed men toward the gratification of passions. According to Hobbes humans are utterly selfish and men constantly try to increase their own power. This behavior causes fear in others, and the conflict continues. Hobbes said that men are equal, but in a different way than you would expect. Hobbes said that men are all equal in that they are the equally the potential murderers of each other.
Hobbes said that without government, everyone would be warring with each other. Humans want peace, he said, but they cannot achieve it because of their self-defeating behavior. To get out of the mess, Hobbes said that we have to appeal to our self-interest, our motivator. Hobbes said that for humans to solve this problem, they must agree to establish a sovereign ruler to wield absolute power; executive, legislative, and judicial. This sovereign, he said, will keep the peace between humans. If the sovereign does not succeed in keeping the peace, then the citizens can withdraw from their contract with him.