Why does Montesquieu believe that there can be no liberty when the legislative and executive powers are united in the same body of magistrates?

Montesquieu argues that liberty cannot exist when legislative and executive powers are united because it leads to the concentration of power, which risks tyranny. He believes that separating these powers ensures a system of checks and balances, preventing any single entity from wielding excessive control over the citizens. When one body holds both powers, it can manipulate laws to serve its interests, undermining individual freedoms and rights. Thus, the division of powers is essential to safeguard liberty and promote justice.