peculiarly so. For in Locke*s theory the mind, ideas as impressions 

 upon the mind, and matter are all not only things out of experience, but 

 things like nothing in experience; things not to be inferred from ex- 

 perience. And in Hyslop's theory the existence of spirits and of the 

 powers ascribed to them are not things which can be experienced, and 

 not things which can be inferred from experience. The materials of 

 Locke's and Hyslop's theories are not matters of experience and are 

 like no things which are matters of experience. These theories are, 

 therefore, complex in the sense that their materials are not presentable 

 in experience, and are to be distinguished from the other theories from 

 the fact that their materials being like nothing in experience, are not to 

 be inferred from it. This latter peculiarity will be considered under 

 the fourth sense of simplicity as applied to theories. 



When thus applied the term may have a third meaning. It may 

 mean only that the reasonings by which the theory is derived are diffi- 

 cult to follow. The reasonings may be sound, but are intricate, and 

 thus tax the strength of the understanding. However, a considera- 

 tion of this sense of simplicity could have but little value, as it is evi- 

 dent that the simplicity of a theory, in this sense, would have no im- 

 portant bearing upon its other qualities. 



There is, however, a fourth meaning which simplicity may have 

 and which we shall find of considerable importance. The term sim- 

 plicity may be used to signify the intelligibility of a theory, and com- 

 plexity to signify unintelligibility. A theory is unintelligible, in whole 

 or in part, when its validity involves the existence of things which we 

 do not know to exist, and the possibility of whose existence we can- 

 not, in the present state of science, understand. An examination of 

 the theories will clarify the meaning of the term simplicity as here used. 

 Darwin's theory and the theory of inorganic evolution are simple in 

 this fourth sense. The concepts involved in both can be clearly set 

 forth to the understanding. Darwin can present as facts in experience 

 all the forces which produce species, and exhibit them in operation. 

 And the astronomer can show to any investigator the changes of tem- 

 perature on the sun and in the stars; and can show the elements of 

 low atomic weight disappearing and those of higher atomic weight ap- 

 pearing as the temperatures fall. Here the materials of the theories 

 are presented as determined facts, and the understanding is not con- 

 cerned with contemplating their possibility. 



In the theory of ions we find the situation, in one part of the 

 theory, somewhat different. The conclusion of the theory is that the 

 number and mechanical arrangements of the ions in the atom explain 

 the periodic law. But it is impossible, in the present state of science, 

 to understand how chemical quality can be dependent upon mechan- 



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