QuestionsWhat is a just society?Power checks power
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Power checks power

Montesquieu 1689 · 1755To prevent abuse, power must check power.
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What it claims

For citizens to be free, power must check power: making the laws, executing them and judging must not be united in the same hands.

Why

Anyone with power tends to abuse it until he meets limits, and only another power sets them.

In short

"Power checks power" is an argument by Montesquieu (The Spirit of the Laws, XI, 4 and 6) on the question "What is a just society?" (answer: liberties). Its claim: For citizens to be free, power must check power: making the laws, executing them and judging must not be united in the same hands. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

The Spirit of the Laws, XI, 4 and 6
Valid with an implicit premise

A good king, a virtuous assembly, a people who vote: is that enough to be free? No, says Montesquieu. Even the best of men, if he holds all power, will end up abusing it. Liberty does not depend on the good intentions of those who govern, but on how power is distributed: making laws, executing them, judging. He never speaks of the "separation of powers": his formula is "power checks power".

P1Every man invested with power is apt to abuse it: he pushes on until he meets limits. Constant experience shows it.
P2There is political liberty only where power is not abused. Being free means being safe: not having to fear another citizen.
P3 · implicit premiseNothing but a power lastingly limits a power: neither the virtue of whoever governs nor his promises. "Virtue itself has need of limits."
P4If the same man or the same body makes the laws, executes them and judges, no other power can check it: "There would be an end of every thing".
Therefore, for citizens to be free, power must check power: making the laws, executing them and judging must not be united in the same hands.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where power is limited, and citizens free, without any other power checking it: for example a king so virtuous that he limits himself. That is exactly what P3 refuses to believe can last.
With P3, the argument is valid. P1 and P2 say: no liberty without limits on power. P3 says: those limits can only come from another power. P4 says: when everything is in the same hands, there is no other power left. P3 is implicit, but it is the heart of the idea: Montesquieu does not rely on the wisdom of rulers, he relies on "the disposition of things". The criticisms mostly target the conclusion: can a divided power still act? And who has the last word?
Analogy. In a match, if the same player scores the goals, keeps the clock and calls the fouls, we can guess who will win. Even if he is honest, no one can check. We do not ask players to be saints: we give the whistle to someone else.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Hobbes 1651 · attacks the conclusion

To divide power is to dissolve it: powers divided mutually destroy each other. England learned it the hard way: it was because people believed power was divided between the King, the Lords and the Commons that the people split and civil war came.

Montesquieu replies
Montesquieu Montesquieu's reply (The Spirit of the Laws, 1748; he does not name Hobbes here)
The legislative body being composed of two parts, one checks the other by the mutual privilege of rejecting. They are both checked by the executive power, as the executive is by the legislative. These three powers should naturally form a state of repose or inaction. But as there is a necessity for movement in the course of human affairs, they are forced to move, but still to move in concert.
The debate goes on (our summary) Montesquieu does not name Hobbes: he describes a mechanism, he does not refute a text. A Hobbesian would say that "forced to move in concert" is a hope more than a proof, and Montesquieu himself admits such a state "will perish" when the legislative power becomes more corrupt than the executive. He would add that in England after 1688 there is still a sovereign with the last word: King, Lords and Commons together in Parliament. Montesquieu's defenders answer that many constitutions with shared powers have lasted more than two centuries. Deadlocks between powers remain a real problem, though.
Rousseau 1762 · attacks the conclusion

Sovereignty is indivisible: the will is either general or it is not. Our political theorists turn the sovereign into "a fantastic being composed of several connected pieces", like jugglers who cut up a child and make it fall down alive. Making the law belongs to the people alone; the rest is only the execution of that law.

no reply
No reply possible: objection raised in 1762, after the author's death (1755).
The debate goes on (our summary) Montesquieu died in 1755, seven years before the Social Contract. The disagreement is narrower than it looks: Rousseau too wants the government (the executive) to be distinct from the sovereign that makes the laws. What he refuses is sharing the law-making power itself, for instance with a chamber of nobles or a king who can say no. For Montesquieu, that sharing is precisely what protects the citizen; for Rousseau, it betrays the general will.
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Louis Althusser 1959 · attacks the conclusion

Behind the balance of powers lies a balance between social classes. In the constitution Montesquieu describes, a chamber of hereditary nobles serves as a "regulating power": his liberty first protects the privileges of those who already have them (Montesquieu, la politique et l'histoire).

Montesquieu replies
Montesquieu What Montesquieu himself provides (The Spirit of the Laws, XI, 6)
As a hereditary power might be tempted to pursue its own particular interests and forget those of the people, wherever it would gain by being corrupted, as in the laws on taxes, it must have only the power of rejecting, not that of resolving.
The debate goes on (our summary) Althusser would answer that the power of rejecting is precisely a veto: the nobles can block any reform touching their privileges. Montesquieu says so himself a few lines earlier: the nobles must form a body "that has a right to put a stop to the enterprizes of the people", to defend privileges that are, he says, "obnoxious to popular envy" and always in danger in a free state (XI, 6). The mechanism "power checks power" can survive without the nobility (modern constitutions kept it while abolishing nobility), but the question remains: who, today, are the powers being balanced?