XXII

Among Prerogative Instances I will place first Solitary Instances. Those are solitary instances which exhibit the nature under investigation in subjects which have nothing in common with other subjects except that nature; or, again, which do not exhibit the nature under investigation in subjects which resemble other subjects in every respect in not having that nature. For it is clear that such instances make the way short, and accelerate and strengthen the process of exclusion, so that a few of them are as good as many.

For instance, if we are inquiring into the nature of color, prisms, crystals, which show colors not only in themselves but externally on a wall, dews, etc., are solitary instances. For they have nothing in common with the colors fixed in flowers, colored stones, metals, woods, etc., except the color. From which we easily gather that color is nothing more than a modification of the image of light received upon the object, resulting in the former case from the different degrees of incidence, in the latter from the various textures and configurations of the body. These instances are solitary in respect to resemblance.

Again, in the same investigation, the distinct veins of white and black in marble, and the variegation of color in flowers of the same species, are solitary instances. For the black and white streaks in marble, or the spots of pink and white in a pink, agree in everything almost except the color. From which we easily gather that color has little to do with the intrinsic nature of a body, but simply depends on the coarser and as it were mechanical arrangement of the parts. These instances are solitary in respect to difference. Both kinds I call solitary instances, or ferine, to borrow a term from astronomers.