My brother David sent me this from the Bastiat Institute. My father was a huge fan of Frederic Bastiat, the acerbic wit of the Austrian economics movement, and always kept a case of his The Law in his car trunk to hand out to anyone he thought might be interested and read it. He probably gave away thousands of copies over his lifetime.
Sentimentality notwithstanding, the problem with this saying is (1) that it is so widely held (implicitly) in traditional neoclassical economic teaching and explicitly in public policy, and (2) that it fundamentally mistakes social contrivances for natural law. The "law" of supply and demand certainly does work in many market contexts (if imperfectly, the modeling of which has built many economists' careers). BUT, those market contexts are built on formal laws and informal norms, on historical trajectories and institutional structures that are cultural, social, human.
It is important that we understand that markets are social contrivances and not the product of natural laws--doing so gives us the freedom to use markets to promote the sort of life we value. Sure, truck and barter is part of the human condition, as Adam Smith phrased it. But we construct just how we go about it. And seeing markets as something we create gives us the power to change them toward the ends we best see fit. In contrast, seeing markets as expressing natural laws allows our leaders to disavow hard moral and political choices onto the moral logics of the market. Industrial policy, regulation, and all the other currently unfashionable ideas should not be seen as misguided attempts to change the inevitable, but as morally neutral tools to orient markets towards ends that we collectively (if I dare use that word) decide are the best.