Our mind grasps the difference of these powers, and it determines not only with what objects they deal, but it also defines their reason or essence. What does it admit that it is easy to distinguish arts according to their reasons, and according to the notions they include? But is it possible to divide corporeal qualities in that manner? Even when one studies the intelligible world, there is room for doubt as to how the different reasons distinguish themselves from each other. It is easy enough to see that white differs from black, but in what does it do so? It is absurd to distinguish being, qualities, and differences by themselves. 18. All the questions we have asked show that we doubtless must seek to discover the differences of the various beings so as to distinguish them from each other, but that it is as impossible as it is unreasonable to inquire what are the differences of the differences themselves. Being of beings, quantities of quantities, qualities of qualities, differences of differences cannot be discovered, but we should wherever possible classify exterior objects either according to their effects or according to salient characteristics. When this is impossible, objects should be distinguished as for instance dark from light green, but how is white distinguished from black? Sensation or intelligence tell us that those things are different without informing us of their reason. Either sensation because its function is not to set forth the reason of things, but only to bring them somehow to our attention. Or intelligence because it discerns things that are simple by intuition without having to resort to rasciocination and limits itself to the statement that something is such or such. Besides, in each one of the operations of intelligence there is a difference, a special distinctive characteristic which enables it to distinguish different things without this difference which is proper to each of the operations of intelligence itself having need to be discerned by the help of some other difference. Some qualities are differences. Are all qualities differences or not? Whiteness, colors, qualities perceived by touch and taste may become differences between different objects, though they themselves be species. And how do the sciences of grammar or of music constitute differences? The science of grammar renders the mind grammatical, and the science of music renders the mind musical, especially if they be untaught, and these thus become specific differences. Besides, we have to consider whether a difference be drawn from the same genus from which the considered things are drawn, or from some other genus.