The manner in which this scientific improvement takes place is by the resolution of phenomena which {vi} are special and complex into others more general and simple. Two cases of this sort may be roughly distinguished, though the distinction between them will not be found on accurate examination to be fundamental. In one case it is the order of the phenomena that is analysed and simplified; in the other it is the phenomena themselves. When the observed facts relating to the weight of terrestrial objects, and those relating to the motion of the heavenly bodies, were found to conform to one and the same law, that of the gravitation of every particle of matter to every other particle with a force varying as the inverse square of the distance, this was an example of the first kind. The order of the phenomena was resolved into a more general law. A great number of the successions which take place in the material world were shewn to be particular cases of a law of causation pervading all Nature. The other class of investigations are those which deal, not with the successions of phenomena, but with the complex phenomena themselves, and disclose to us that the very fact which we are studying is made up of simpler facts: as when the substance Water was found to be an actual compound of two other bodies, hydrogen and oxygen; substances very unlike itself, but both actually present in every one of its particles. By processes like those employed in this case, all the variety of substances which meet our senses and compose the planet on which we live, have been shewn to be {vii} constituted by the intimate union, in a certain number of fixed proportions, of some two or more of sixty or seventy bodies, called Elements or Simple Substances, by which is only meant that they have not hitherto been found capable of further decomposition. This last process is known by the name of chemical analysis: but the first mentioned, of which the Newtonian generalization is the most perfect type, is no less analytical. The difference is, that the one analyses substances into simpler substances; the other, laws into simpler laws. The one is partly a physical operation; the other is wholly intellectual. Both these processes are as largely applicable, and as much required, in the investigation of mental phenomena as of material. And in the one case as in the other, the advance of scientific knowledge may be measured by the progress made in resolving complex facts into simpler ones.