BOOK: I1.1J PREROGATIVE INSTANCES. 4 83 



nature of the subject ; 5. Of the prerogative natures with respect 

 to investigation, or of what should be the first or last objects of 

 our research ; 6. Of the limits of investigation, or a synopsis of 

 all natures that exist in the universe; 7. Of the application to 

 practical purposes, or of what relates to man ; 8. Of the prepa 

 rations for investigation ; 9. And lastly, of the ascending and 

 descending scale of axioms. p 



XXII. Amongst the prerogative instances we will first men 

 tion solitary instances. Solitary instances are those which 

 exhibit the required nature in subjects that have nothing in 

 common with any other subject than the nature in question, or 

 which do not exhibit the required nature in subjects resembling 

 others in every respect except that of the nature in question ; 

 for these instances manifestly remove prolixity, and accelerate 

 and confirm exclusion, so that a few of them are of as much 

 avail as many. 



For instance, let the inquiry be the nature of colour. Prisms, 

 crystalline gems, which yield colours not only internally but on 

 the wall, dews, &c., are solitary instances ; for they have nothing 

 in common with the fixed colours in flowers and coloured gems, 

 metals, woods, &c., except the colour itself. Hence we easily 

 deduce that colour is nothing but a modification of the image of 

 the incident and absorbed light, occasioned in the former case 

 by the different degrees of incidence, in the latter by the various 

 textures and forms of bodies. 1 These are solitary instances as 

 regards similitude. 



Again, in the same inquiry the distinct veins of white and 

 black in marble, and the variegated colours of flowers of the 

 same species, are solitary instances ; for the black and white of 

 marble, and the spots of white and purple in the flowers of the 

 stock, agree in every respect but that of colour. Thence we 

 easily deduce that colour has not much to do with the intrinsij 

 natures of any body, but depends only on the coarser and as it 

 were mechanical arrangement of the parts. These are solitary 

 instances as regards difference. We call them both solitary or 

 wild, to borrow a word from the astronomers. 



XXIII. In the secc/nd rank of prerogative instances we will 

 consider migrating instances. In these the required nature 

 passes towards generation, having no previous existence, or to 

 wards corruption, having first existed. In each of these divi 

 sions, therefore, the instances are always twofold, or rather it is 

 one instance, first in motion or on its passage, and then brought 



P Of these nine geneial heads no more than the hrst is prosecuted by 

 the author. 



i This very nearly approaches to Sir I. Newton s discovery of the 

 decomposition of light by the prism. 



