238 JAMES CLERK MAXWELL. [CHAP. VIII. 



nate with the three known ones, but assert the objective truth 

 that points may differ in position by the independent 

 variation of three variables. Here, therefore, we have a 

 real analogy between the constitution of the intellect and 

 that of the external world. 



" With respect to time, it is sometimes assumed that the 

 consecution of ideas is a fact precisely the same kind as the 

 sequence of events in time. But it does not appear that 

 there is any closer connection between these than between 

 mental difference, and difference of position. No doubt it is 

 possible to assign the accurate date of every act of thought, 

 but I doubt whether a chronological table drawn up in this 

 way would coincide with the sequence of ideas of which we 

 are conscious. There is an analogy, but I think not an 

 identity, between these two orders of thoughts and things. 

 Again, if we know what is at any assigned point of space 

 at any assigned instant of time, we may be said to know all 

 the events in Nature. We cannot conceive any other thing 

 which it would be necessary to know ; and, in fact, if any 

 other necessary element does exist, it never enters into any 

 phenomenon so as to make it differ from what it would be 

 on the supposition of space and time being the only neces- 

 sary elements. 



" We cannot, however, think any set of thoughts without 

 conceiving of them as depending on reasons. These reasons, 

 when spoken of with relation to objects, get the name of 

 causes, which are reasons, analogically referred to objects 

 instead of thoughts. When the objects are mechanical, or 

 are considered in a mechanical point of view, the causes are 

 still more strictly denned, and are called forces. 



" Now if we are acquainted not only with the events, 

 but also with the forces, in Nature, we acquire the power of 

 predicting events not previously known. 



" This conception of cause, we are informed, has been 

 ascertained to be a notion of invariable sequence. No doubt 

 invariable sequence, if observed, would suggest the notion of 

 cause, just as the end of a poker painted red suggests the 

 notion of heat, but although a cause without its invariable 



