INTRODUCTION. 33 ' 



The method of induction here laid down is applicable to all investi- ! 

 gations where experience is the guide, whether in the moral or natural I 

 world. 



It is obvious that all facts, even supposing them truly and accurately 

 recorded, are not of equal value in the discovery of truth. Some of 

 them show the thing sought for in its highest degree, others in its lowest; 

 some show it simple and uncombined, while others are confused with a 

 variety of circumstances. Some facts are easily interpreted, others are 

 very obscure, and are understood only in consequence of the light thrown 

 on them by the former. This led Bacon to consider the comparative 

 value of facts as means of discovery. He enumerates twenty-seven dif- 

 ferent species ; but we shall satisfy ourselves here with noticing a few of 

 the most important of them : 



1. Instantice solitaries are examples of the same quality existing in two 

 bodies, which have nothing else in common ; or of a quality differing in 

 two bodies, which are in all other respects the same. 



2. The instantice migrantes exhibit some nature or property of bodies 

 passing from one condition to another, either from less to greater, or 



( from greater to less. Thus, glass while entire is colorless, but becomes 

 > white when reduced to powder. 



3. The instantice ostensivce show some particular nature in its highest 

 state of power or energy. In this way the thermometer shows the ex- 

 pansive power of heat, and the barometer the weight of air. 



4. The instcntia analogies consist of facts between which an anal- 

 ogy or'ressmU-r.ce is visible in some particulars, notwithstanding great 

 diversity in all the rest. Such are the telescope and microscope in 

 works of art, compared with the eye in the works of nature. 



5. The instantice crucis is the division of this experimental logic | 

 which is the most frequently resorted to in the practice of inductive in- 

 vestigation. When, in such an investigation, the understanding is, as it 

 were, placed in equilibrio between two or more causes, each of which 

 accounts equally well for the appearances, so far as they are known, 

 nothing remains but to look out for a fact which can be explained by the <J 

 one of these causes, and not by the other. If such a fact can be fouDd, 

 the uncertainty is removed, and the true cause becomes apparent. Such ' 



VOLi. I. 3 



