The study of psychology with Plato stands in a fourfold relation to his general theory of the world. The dialectic method, without which Nature would remain unintelligible, is a function of the soul, and constitutes its most essential activity; then soul, as distinguished from body, represents the higher, supersensual element of existence; thirdly, the objective dualism of reality and appearance is reproduced in the subjective dualism of reason and sense; and lastly, soul, as the original spring of movement, mediates between the eternal entities which are unmoved and the material phenomena which are subject to a continual flux. It is very characteristic of Plato that he first strains an antithesis to the utmost and then endeavours to reconcile its extremes by the interposition of one or more intermediate links. So, while assigning this office to soul as a part of the universe, he classifies the psychic functions themselves according to a similar principle. On the intellectual side he places true opinion, or what we should now call empirical knowledge, midway between demonstration and sense-perception. Such at least seems to be the result reached in the _Theaetêtus_ and the _Meno_. In the _Republic_ a further analysis leads to a somewhat different arrangement. Opinion is placed between knowledge and ignorance; while the possible objects to which it corresponds form a transition from being to not-being. Subsequently mathematical reasoning is distinguished from the higher science which takes cognisance of first principles, and thus serves to connect it with simple opinion; while this again, dealing as it does with material objects, is related to the knowledge of their shadows as the most perfect science is related to mathematics.[138]