In the first stage of his career, Hall's particular interest was to draw on the social disciplines to increase knowledge of criminal law. In the course of his interdisciplinary studies, he learned that one could not simply lift factual data and theories from social science and apply them to legal problems. In the second stage, Hall concentrated on legal concepts of jurisprudence and moral philosophy. His work led to the conclusion that criminology should be regarded as synonymous with the sociology of criminal law, and that its subject matter should concern itself with actions and facts at the varying levels of abstraction embodied by rules, principles, and doctrines. On the basis of his experiences, Hall notes the value of oral dialog among scholars and suggests the need for an institute to further interdisciplinary study. He concludes that theory and practice must go hand in hand. Practice unenlightened by theory leads to pedestrian results, while theories not tested in practical reserach remain figments of speculation.