theory is, however, of little value ; for the amount of data involved in 

 a theory has no significant bearing upon its important qualities. All 

 the theories, here examined, are simple in this respect, except, perhaps, 

 the theory of ions. Each of the theories is made up of few concepts 

 and of few and simple relations between them. Darwin's theory in- 

 volves merely individual variations, inheritance, and natural selection. 

 Hyslop's deals with souls, mediums, and communications through the 

 mediums. The theory of inorganic evolution considers merely changes 

 of temperature and corresponding changes in elemental structure. The 

 theory that matter is electrical simply compares the essential qualities 

 of electricity and matter. The theory of ions is a little more complex 

 in this respect, involving a consideration of the different kinds of ions 

 and their properties ; the number and arrangement of ions in the atom ; 

 and the relationship between the structure of the atom and its chemical 

 properties. It will be seen that different as these theories are in other 

 important respects, they all contain but few concepts and but few 

 relations between them, and are all in this sense comparatively simple. 



Secondly, the term simple may be used to designate that quality 

 of a theory which depends upon the directness with which its data 

 can be presented, and the number of other theories involved in, or 

 whose validity is implied in the theory in question. In illustration, 

 Darwin's theory is simple in this respect: individual variation, in- 

 heritance, and natural selection are processes which can be presented 

 directly in experience. The theory involves, however, inferences as to 

 the length of time through which organic life has existed on the earth, 

 and the nature of past geological conditions. In this respect the 

 theory is in this second sense complex; it is concerned with existences 

 which cannot be directly presented in experience. In the theory of in- 

 organic evolution, changes of temperature on the earth, sun, and stars 

 are inferred. Also the existence and non-existence of elements are 

 inferred from light emissions. Its materials are not directly presented 

 in experience, and in this sense it is complex. The theory also in- 

 volves inference as to past astronomical changes. 



Concerning the theory that matter is electrical: electrical qualities 

 are manifestations in experience and so also are the qualities of mat- 

 ter. But this theory rests upon the theory of ions, the materials of 

 which are inferred existences. This latter theory deals entirely with 

 inferred existences. The ions, their properties and arrangements are 

 all inferential facts. The theory that matter is electrical and the 

 theory of ions are both then comparatively complex in the sense that 

 the materials with which they deal are not facts of experience, but are 

 inferences from it. 



Locke's theory and Hyslop's also are complex in this sense; and 



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