The job of legal scholars is to describe the structure and coherence of norms Many philosophers of law presume it is impossible to derive norms from facts. All that nature can offer is informatie, not prescription, it is said. However, such a rigourous divide will cause jurisprudence to cut itself off form new discoveries in empiric sciences. On what terms is a biological theory of law feasible? Could it meet the objections against natural law theory? Would it be possible to justify our deeds by referring to biological mechanisms? Aren't we actually doing it all the time? The author argues that biological mechanims can be morally obligatory, because not living by them is tantamount ot failure to live in a group, with the result that underlying genes will not spread. We feel we ought to live according to group moral and rules. 'Reason' and 'normative thinking' are the result of biological processes; there are no external sources.