What phrase embodies the idea of self-government in the Constitution?

The phrase "We the People" embodies the idea of self-government in the Constitution because it explicitly states that the authority of the government is derived from the people themselves. This phrase emphasizes that the government is established by and for the citizens, indicating that all power and legitimacy stem from the collective agreement and will of the populace.

This notion is foundational in democratic societies, as it signifies that the people have the right to govern themselves and have a say in the formation and function of their government. In contrast, the other phrases, while significant and related to democratic principles, do not encapsulate the concept of self-government as directly. "Of the People, By the People, For the People" reflects a similar sentiment but is not part of the Constitution itself; it's from Abraham Lincoln's Gettysburg Address and reinforces the principles already established by "We the People." Meanwhile, "In Order to form a more perfect union" and "To secure the Blessings of Liberty" express intents and goals of the government rather than directly stating the source of its power, which "We the People" does effectively.