It should be observed that in determining the particulars of external duty the Stoics to some extent used the notion `nature' in a different way: they tried to derive guidance from the complex adaptation of means to ends exhibited in the organic world. But since in their view the whole course of the Universe was both perfect and completely predetermined, it was impossible for them to obtain from any observation of actual existence a clear and consistent principle for preferring and rejecting alternatives of conduct: and in fact their most characteristic practical precepts show a curious conflict between the tendency to accept what was customary as `natural,' and the tendency to reject what seemed arbitrary as unreasonable.