The resolution of complex Facts into precise and measured partial Facts, we call the 'Decomposition of Facts'. This process is requisite for the progress of science, but does not necessarily lead to progress. The Conceptions by which Facts are bound together, are suggested by the sagacity of discoverers. But a supply of appropriate hypotheses cannot be constructed by rule, nor without inventive talent. The truth of tentative hypotheses must be tested by their application to facts. The discoverer must be ready, carefully to try his hypotheses in this manner, and to reject them if they will not bear the test, in spite of indolence and vanity.