Language & WorldMental model
Why nobody may make, enforce and judge laws at once.
Montesquieu described three powers in 1748: the legislature (parliament) passes laws, the executive (government, agencies, police) carries them out, and the judiciary (courts) checks whether both are lawful. Each can restrain the others: courts strike down laws, parliaments topple governments. The model sits in almost every modern constitution, applied with varying strictness.
Useful: Whenever power concentrates, ask: who still checks it, and can they lose by doing so?
Source: en.wikipedia.org