Pope John Paul answered this question nicely in his encyclical Veritatis Splendor. He said the natural law of moral theology "receives this name not because it refers to the nature of irrational things but because the reason which promulgates it is proper to human nature" (VS 42). The moral law is called natural law because it is based on our nature as rational beings. It is not based on the nature of irrational beings, such as animals, plants, or inanimate matter. When scientists refer to the laws of nature, they mean physical laws such as the law of gravity or the laws of thermodynamics.