When seeking to solve the many problems of our day the republican talking heads, ‘tea party’ folk, as well as many of your average conservatives tell us repeatedly to “go back to the Founding Fathers and the Constitution” to reestablish a just and economically stable society. They encourage us to do so because many of the Founders were brilliant men who knew a great deal about politics and government, and because they were the ones that more or less conceived this nation. They were certainly great men. But a contract written by these men (or any group of men) cannot be the ultimate standard for law; as we shall see.
The Founders were greatly influenced by the Enlightenment period of the 18th century. The Enlightenment was a repudiation of Religion, and resulted in an embracing of atheistic, Unitarian, and deistic ideologies as well as a return to ancient pagan Greek philosophies. And with that “enlightenment” returned the Greek idea of “Natural law” . We can see its influence in the opening paragraph of the United States’ Declaration of Independence:
“When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the Powers of the earth, the separate and equal station to which the Laws of Nature and of Nature’s God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.” (Emphasis added.)
“Natural law” is the idea that man can attain to a law/standard by,
a) By following his conscience/common sense, or
b) By digging deep into the recesses of one’s own noggin. (Which as a system is called Rationalism )
“Morals are too essential to the happiness of man to be risked on the uncertain combinations of the head. She [Nature] laid their foundation therefore in sentiment…” –Thomas Jefferson 
“My own mind is my own church.” –Thomas Paine 
“To understand political power right, and derive it from its original, we must consider what state all men are naturally in, and that is, a state of perfect freedom to order their actions, and dispose of their possessions, and persons as they think fit, within the bounds of the law of nature, without asking leave, or depending on the will of any other man.” –John Locke 
Is this “Natural law” idea legitimate? First, we must note that you can never come to an “ought” from what “is” which is what this philosophy tries to do. For the reader who doesn’t know the difference between is and ought, we will consider the difficulties of deriving norms (or what ought be) from nature (what is). What if I have a “natural” feeling that since Mr. X really ticked me off, it should be legal for me to murder him, should it be? But his intuition tells him that it should be illegal for me to murder him. Whose intuition, or “natural law” do we go by?
Or, should we follow the arbitrary brilliance of the Enlightenment thinker, Francis Hutcheson, who thought, “that action is best which procures the greatest happiness for the greatest numbers.”?  What is “the greatest happiness”? Happiness is not a measurable thing; how can you determine which is “more” happiness, really great happiness that lasts for a moment, or moderate happiness that lasts a lifetime? Or what if my happiness comes by murdering Mr. X? Natural law is a failure, and can never be the foundation of a society.
However, the Founders were also greatly influenced by the early Puritans and men of the Protestant Reformation, like John Calvin and John Knox who recognized the Word of God was the only rule for the church, family, and civil government; (a principle commonly called sola Scriptura) as Paul states in II Timothy 3:16, “All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:” (see also, Colossians 2:3-8, Isaiah 8:20, & John 10:35).
The Founders, having been greatly influenced by both the Puritans and the Enlightenment decided to blend Biblical Law with “Natural law”, which is like blending your steak with excrement, or your wine with urine. This has defiled our whole law-system.
 This idea is seen in the work of Sophocles, Oedipus the King.
 See Biblical View of Knowledge, Part 3: https://reformedinquisitor.wordpress.com/2011/03/22/the-christian-view-of-knowledge-part-3-rationalism/).
 Thomas Jefferson 1743-1826, Author of the Declaration of Independence, in Writings, p. 874 (1786)
 Thomas Paine 1737–1809, Opening pages of his book, Age of Reason (1794)
 John Locke, Second Treatise, Chapter 2.
 Francis Hutcheson 1694-1746, Scottish philosopher in Inquiry Concerning Moral Good and Evil, sec. 3 (1725)