6 THE POPULAR SCIENCE MONTHLY. 



Thus, both doubt and belief have positive effects upon us, though 

 very different ones. Belief does not make us act at once, but puts us 

 into such a condition that we shall behave in a certain way, when the 

 occasion arises. Doubt has not the least effect of this sort, but stim- 

 ulates us to action until it is destroyed. This reminds us of the irri- 

 tation of a nerve and the reflex action produced thereby ; while for 

 the analogue of belief, in the nervous system, we must look to what 

 are called nervous associations for example, to that habit of the 

 nerves in consequence of which the smell of a peach will make the 

 mouth water. 



IV. 



The irritation of doubt causes a struggle to attain a state of belief. 

 I shall term this struggle inquiry, though it must be admitted that 

 this is sometimes not a very apt designation. 



The irritation of doubt is the only immediate motive for the strug- 

 gle to attain belief. It is certainly best for us that our beliefs should 

 be such as may truly guide our actions so as to satisfy our desires ; 

 and this reflection will make us reject any belief which does not seem 

 to have been so formed as to insure this result. But it will only do so 

 by creating a doubt in the place of that belief. With the doubt, 

 therefore, the struo-o-le begins, and with the cessation of doubt it ends. 

 Hence, the sole object of inquiry is the settlement of opinion. We 

 may fancy that this is not enough for us, and that we seek, not merely 

 an opinion, but a true opinion. But put this fancy to the test, and it 

 proves groundless ; for as soon as a firm belief is reached we are en- 

 tirely satisfied, whether the belief be true or false. And it is clear 

 that nothing out of the sphere of our knowledge can be our object, for 

 nothing which does not affect the mind can be the motive for a mental 

 effort. The most that, can be maintained is, that we seek for a belief 

 that we shall think to be true. But we think each one of our beliefs 

 to be true, and, indeed, it is mere tautology to say so. 



That the settlement of opinion is the sole end of inquiry is a very 

 important proposition. It sweeps away, at once, various vague and 

 erroneous conceptions of proof. A few of these may be noticed here. 



1. Some philosophers have imagined that to start an inquiry it 

 was only necessary to utter a question or set it down upon paper, and 

 have even recommended us to begin our studies with questioning 

 everything ! But the mere putting of a proposition into the interrog- 

 ative form does not stimulate the mind to any struggle after belief. 

 There must be a real and living doubt, and without this all discussion 

 is idle. 



2. It is a very common idea that a demonstration must rest on 

 some ultimate and absolutely indubitable propositions. These, ac- 

 cording to one school, are first principles of a general nature; accord- 

 ing to another, are first sensations. But, in point of fact, an inquiry, 



