Bill Whittle's Part IV of the explanation of Tea Party/conservative beliefs:
Natural law means the right to swing your fist ends where my nose begins.
Political law means that the "authorities" decide you need to be have your limbs fettered and bound because you might swing your fists.
John Locke wrote much about Natural Law in his Second Treatise of Government, and he starts by addressing man kind in a condition of nature - wherein man is free, and left to his own devices. There is no government to determine what an individual can or cannot do - his actions are subject only to the limitations of the individual's imagination and capability. In a state of perfect freedom, an individual is subject only to the triumphs and tragedies and vicissitudes of life. Government exists as a compact between indivuduals to provide for mutual safety and improvment.
Often times, once government has established itself as a power, be it the tribal chief, the invading warlord, or the gerrymandered-district eternal senator; the government as an institution is simply a tool of power for a tyrant - not a tool for groups of individuals to provide for their common defense and well-being.
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 to 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. Prudence, indeed, will dictate that governments long established should not be changed for light and transient causes; and accordingly all experience hath shown that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same object evinces a design to reduce them under absolute despotism, it is their right, it is their duty, to throw off such government, and to provide new guards for their future security.
Governments exist by the consent of the governed, and are rejected by those who are governed, but no longer wish to be. Contracts exist by consent of those involved, and are to be honored.
Government existing at the behest of the governors - the ruling class - is tyranny.