Popular sovereignty is the idea that government's power should be determined by the people. The Declaration asserts that to secure their individual rights, the people institute governments for themselves -- that governments derive "their just powers from the consent of the governed."
The same phrase within the Declaration focuses on the idea of a social contract - that our agreement to live under a government is an implicit pact between the governors and the governed. Social contract theory was argued by English philosophers such as Thomas Hobbes and John Locke in the 17th century. American founding fathers took a number of their ideas from the political philosophy of John Locke. Locke's Second Treatise on Civil Government put forth his social contract theory and design for a representative form of government.
We haven't yet addressed natural rights. The strong assertion that all human beings have inherent natural rights is asserted in the most famous phrase from the Declaration of Independence: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
