44 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



a perception of the connection of two things ; and when 

 an infant handles a solid body, it concludes in an incipient 

 degree that his impressions of vision and touch belong to 

 the same object. Correct perceptions of distance and 

 magnitude are originally obtained by the combined aid of 

 the intellect and experience, and all our more abstruse 

 ideas require the exercise of reason. We often employ 

 our reason also in forming perceptions. 



Ideas are of many kinds, and may be divided into 

 true and false, real and fanciful, simple and complex, 

 strong and feeble, distinct and indistinct, complete and 

 incomplete, adequate and inadequate, relative and abso- 

 lute, disorderly, confused, axiomatic, abstract, ultimate, 

 essential, immediate and mediate, qualitative and quan- 

 titative, &c. ; and some of these are treated of in the 

 chapter on ' Scientific Terms.' 



An idea, associated with almost any common name, 

 may differ in kind, and its implicit contents differ in 

 quality. It may either be what is logically termed in 

 extension or intension. Thus we may either think of a 

 steamship in extension as any single vessel propelled by 

 the expansion of steam, or in intension as any one of the 

 individual steamships that we are acquainted with. In 

 the former case our idea includes a large number of 

 objects, and in the latter a large number of marks or 

 attributes ; and the completeness of our idea depends upon 

 our clearly conceiving it in both these aspects. As, how- 

 ever, the human mind can only think of a few marks at a 

 time, and cannot realise those with a vividness equal to 

 that excited by the original object, there is a limit to the 

 degree of clearness of every mental conception ; the more 

 objects or ideas also we perceive at once, the less we per- 

 ceive of each. 



Scientific ideas may be either of real or possible 



