THE METAPHYSICS OF SCIENCE. 363 



cognized internal phenomena. Hence the certainty of 

 external phenomena is conditioned on the validity of this 

 belief. External phenomena, therefore, cannot become so 

 immediately, even if they are so certainly, the materials 

 of valid knowledge, as those phenomena which arise in 

 the mental field. 



Among the phenomena of consciousness we have to 

 make, therefore, the following discriminations : First, 

 Mental states, or psychic modes, without regard to their 

 sources, occasions or coordinations to any other facts than 

 mental states; Secondly, Those among the mental states 

 w^hicli we irresistibly refer to external phenomena as their 

 correlates and causes. But there is also a tJiird category 

 of mental states, or inner perceptions, which we irresistibly 

 refer to abstract and necessary truths. 



The truths thus cognized as having a necessary, uni- 

 versal and eternal existence are truths concerninof neces- 

 sary being and necessary relations. Space and time are 

 existences which must be held necessary in the same sense 

 as other truths are necessary; and the relations of por- 

 tions of them are relations of quantit}^, which are formu- 

 lated in well-known axioms and theorems, embraced among 

 the necessary truths which stand as correlates to the third 

 class of mental states. Other truths are the inseparable- 

 ness of quality and substance, attribute and being, effect 

 and cause, order and intelligence, continuity of existence, 

 universality of law, ultimate unity and ultimate primordi- 

 ality of existence. Some of these principles have generally 

 been omitted from enumerations of necessary truths; and 

 the reader, if he think proper, can omit them here, as the 

 main purpose is simply to adduce illustrations, and not 

 to establish a catalogue. 



