Enneads VI.3: Plotino's Own Sense-Categories
Difference in Effects Is Limited to the Intelligibles
A further possibility is that these things should be classified according to their effects, and that it is reasonable to do so with invisible entities, such as sciences; but this would not be applicable to sense-objects. When indeed we divide sciences by their effects, and when, in general, we classify them according to the powers of the soul, by concluding from the diversity of their effects that they differ, 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). Let us 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 ratiocination, 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.