Having read Article I, Article II and Article III, step back and see the design: three articles, three branches, three kinds of power. This is Montesquieu's idea from the social contract thinkers, built into a structure.
Why it works
Madison stated the fear plainly in Federalist No. 47: "The accumulation of all powers, legislative, executive, and judiciary, in the same hands, whether of one, a few, or many, and whether hereditary, self-appointed, or elective, may justly be pronounced the very definition of tyranny." Critics answered that the Constitution mixed the branches: the President vetoes laws, the Senate approves appointments. Madison replied that Montesquieu never meant the branches should have nothing to do with each other, only that no one branch should hold the whole of another's power. In Federalist No. 48 he added that written lines, "parchment barriers," are not enough: each branch needs ways to defend itself. That leads to checks and balances.
Key idea
Separation of powers gives lawmaking to Congress (Article I), carrying out the laws to the President (Article II) and deciding cases to the courts (Article III), so no one branch holds all power. The branches are separate institutions that share some powers.
Worked example: Which branch?
An agency of the executive branch writes detailed rules for a new law. A court later decides whether one rule goes beyond the law. Name the branch doing each job.
- Writing rules to carry out a statute is executing the law: the executive branch.
- Deciding whether the rule fits the law is judging: the judicial branch.
Answer: Executive, then judicial.
Worked example: Separate or shared?
Lawmaking belongs to Congress. Name one way the President takes part in it anyway.
- Article I, Section 7: every bill goes to the President.
- The President may sign or veto it; the veto can be overridden by two-thirds of each house.
Answer: The veto (and signing bills); lawmaking is shared.
Common mistake
Thinking separation means the branches never share a task. They are separate institutions, with separate people (a member of Congress cannot hold another federal office at the same time), that share powers: lawmaking (the veto), appointments (Senate consent), treaties. Check: ask "who holds the office?" (always separate) and "who takes part in the decision?" (often more than one).