“When the legislative and executive powers are united in the same person, or in the same body of magistracy, there can be no liberty.”Montesquieu · Despre spiritul legilor · 1748 · Cartea XI, Capitolul 6
Freedom is lost when the same entity both makes the laws and enforces them.
Montesquieu shows that if the same group makes and enforces laws, it can abuse power unchecked. No one can stop it from creating laws that serve its own interests. For example, a king who both makes and executes laws can impose taxes at will, with no one to challenge him.
Why it matters This separation of powers is the foundation of modern democracies. We use it when we demand that the government not control both the judiciary and the parliament.