Robert E. Goodin is Distinguished Professor of Philosophy and of Social & Political Theory in the Research School of Social Sciences at Australian National University. This talk was recorded January 16, 2008 as the 2007-2008 John Dewey Lecture on Jurisprudence.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. For law to do what law is supposed to do, which is to be action-guiding, people need to be able to intuit without detailed investigation what the law is for most common and most important cases of their conduct, and to intuit when their intuitions are likely to be unreliable and hence that they need to investigate further what the law actually is.