In science, an empirical relationship is a relationship or correlation based solely on observation rather than theory. An empirical relationship requires only confirmatory data irrespective of theoretical basis. Sometimes theoretical explanations for what were initially empirical relationships are found, in which case the relationships are no longer considered empirical. Thus correlation is not causation but in some cases can be found to result from it. Other times the empirical relationships are merely approximations, often equivalent to the first few terms of the Taylor series of the "real" answer (though in practice these approximations may be so accurate it is difficult to tell they're approximations). Still other times the relationships may later be found to only hold under certain specific conditions, reducing them to special cases of more general relationships.