484 JfOVUM OttOAKtJtf. [flOOK 11 



to tiie opfJOsite dOticlusion. These instances not Only hasten and 

 confirm exclusion, but also reduce aiBrmation, or the form itself, 

 to a narrow compass ; for the form must be something conferred 

 by this mifrration, or, on the contrary* removed and destroyed by 

 it; an.4 although all exclusion advances affirmation, yet this 

 takes place more directly in the same than in different subjects ; 

 but if the form (as it is quite clear from what has been advanced) 

 exhibit itself in one subject, it leads to all. The more simple 

 the migration is, the more valuable is the instance. These mi- 

 gratin<ij instances are, moreover, very useful in practice, for 

 since they manifest the form, coupled with that which causes or 

 destroys it, they point out the riglit practice in some subjects, 

 and thence there is an easy transition to those with which tlicy 

 are most allied. There is, however, a degree of danger which 

 demands caution, namely, lest they should refer the form too 

 much to its efficient cause, and imbue, or at least tinge, the un- 

 derstanding with a false notion of the form from the appearance 

 of such cause, which is never more than a vehicle or conveyance 

 of the form. This nay easily be remedied by a proper applica- 

 tion of exclusion. 



Let us then give an example of a migrating instance. .Let 

 whiteness be the required nature. An instance which passes 

 towards generation is glass in its entire and in its powdered 

 state, or water in its natural state, and when agitated to froth ; 

 for glass when entire, and water in its natural state, arc transpa- 

 rent and not white, but powdered glass and the froth of water 

 are white and not transparent. We must inquire, therefore, 

 what has happened to the glass or water in the course of this 

 migration ; for it is manifest that the form of whiteness is con- 

 veyed and introduced by the bruising of the glass and the agita- 

 tion of the water; but nothing is found to have been introduced 

 but a diminishing of the parts of the glass and water and the 

 insertion of air. Yet this is no slight progress towards dis- 

 covering the form of whiteness, namely, that two bodies, in 

 themselves more or less transparent (as air and water, or air and 

 glass), when brought into contact in minute portions, exhibit 

 whiteness from the unequal refraction of the rays of light. 



But here we must also give an example of the danger and 

 caution of whicli we spoke ; for instance, it will readily occur to 

 an understanding perverted by efficients, that air is alwa> a 

 necessary for producing the form of whiteness, or that white- 

 ness is only generated by transparent bodies, which suppositions 

 are both false, and proved to be so by many exclusions ; nay, it 

 M ill rather appear (without any particular regard to air or the 

 like), that all bodies which are even in such of their parts as 

 affect the sight exhibit transparency, those which are uneven 

 vad of simple texture whiteness, those which are uneven and of 



