INTRODUCTION. 



6. We are said to explain a phenomenon, when 

 we shew it to be necessarily included in some phe- 

 nomenon or fact already known, or supposed to be 

 known ; and we consider one phenomenon as the 

 cause of another, when we conceive the existence 

 of the latter to depend on some force or power resi- 

 ding in the former. 



7. A fact assumed in order to explain a certain 

 set of appearances, and having no other evidence 

 of its reality, but the explanation which it gives of 

 those appearances, is called a HYPOTHESIS. 



8. If the number of appearances explained is very 

 great, or if the explanation is founded on facts 

 known to be true from evidence independent of 

 those appearances, the explanation is called a 



THEORY. 



A theory is often nothing else but a contrivance for 

 comprehending a certain number of facts under one 

 expression. 



In the explanation of natural appearances, and in all 

 inductive reasonings, Facts, though equally certain, 

 may not be of the same value for the discovery of 

 truth. BACON has classified facts, and explained 

 their peculiar advantages as instruments of investiga- 

 tion, in the 2d book of the NOVUM ORGAXUM. 



9. When one system of events or appearances is 

 similar to another, and when we infer that the 



(3) A 3 onuses 



