“Ignorance of the law is no excuse, or so we are told. But why on earth not? The statute books run to hundreds of volumes. How can an ordinary citizen know what all is in them? The best way might be for law (at least in its wide-scope duty-conferring aspects) to track broad moral principles that ordinary citizens can know and apply for themselves. In contrast to more high-minded and deeply principled arguments, this epistemic argument for legal moralism is purely pragmatic – but importantly so.” 

And video.