Philosophy is not any different than theroretical physics, except that physics can be proven with math. Einstein had just as wild imagination as John Locke and Rene Descartes. Unfortualey philosophy can't be proven with numbers. The Bill of Rights stems from a lot on English common law and also the graddaddy of all Bill of Rights, the Magna Charta. I donn't think that the Enlightenment Philosophers invented a lot of new things, but rather put the pieces together from existing ideas. Just as Issac Newton K.B.E. didn't invent light, but discovered the principles of light.
Natural Rights are an extension of Free Agency. You can't be free unless you have the ability to act. However, John Locke said that Natural Rights included the right to life, liberty and property. The Founders didn't believe in a right to property, (i.e. taxes) so they substitued "Happiness". As for life and liberty, well... tell a wilderbeast being eaten by a lion that he has the right to life.
