A general flat minimum-wage law for all industry is permissible, but I do not think that it is a particularly wise method of achieving the end. I know much better methods of providing a minimum for everybody. But once you turn from laying down a general minimum for all industry to decreeing particular and different minimum for different industries, then, of course, you make the price mechanism inoperative, because it is no longer the price mechanism which will guide people between industries and trades.
