I think that at least for the purposes of our government, they were expressed when we declared 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. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
So one could say that the purpose of government is to preserve the right of people to express themselves on matters of reducing suffering as the communally understand that to be, how we self define, life, liberty and happiness.
Furthermore, as an aside, we have in place now a government that does not correspond to the majority of the people and therefore will not stand. Our conscious understanding of the inalienable rights we are entitled to is evolving through time.