 Traditionally, philosophers of science took successful scientific explanations to result from derivation from laws of nature (see the entries on  laws of nature  and  scientific explanation).  On this deductive-nomological account (Hempel and Oppenheim 1948), an explanation of particular observation statements was analyzed as subsumption under universal (applying throughout the universe), general (exceptionless), necessary (not contingent) laws of nature plus the initial conditions of the particular case. Philosophers of biology have criticized this traditional analysis as inapplicable to biology, and especially molecular biology.