what is the thesis of federalist 51?

This Federalist Paper, written by James Madison focuses on the need for checks and balances in government while reminding people in government that separation of powers is critical to balance any one person or branch whose ambition is overwhelming against someone who is also overly ambitious.

Actually the exact thesis is stated in the essay itself. It is as follows: "without presuming to undertake a full development of this important idea, I will hazard a few general observations which may perhaps place it in a clearer light, and enable us to form a more correct judgment of the principles les and structure of government planned by the convention".