As an example of this tendency, Romer cites the work of the University of Chicago economist Robert Lucas, who, in his 2009 paper “Ideas and Growth,” dismisses the role that books or blueprints can play in driving growth. “Some knowledge can be ‘embodied’ in books, blueprints, machines, and other kinds of physical capital, and we know how to introduce capital into a growth model,” Lucas argued, “but we also know that doing so does not by itself provide an engine of sustained growth.”