<span>The "door" is the amendment process, which allows for changes to the constitution. They included it because things change and government must be capable of changing or it will stagnate and die. They did not make it an easy process, because the constitution was largely a very good document, and the limits that it placed on the government were very important, as most of them continue to be today.</span>
The Spirit of the Laws was the book that described a version of Roman government that used the separation of government's power into independent branches as a main principle.
