Since degrees of breadth, that is, continuous degrees, are like gradations of light to shade, heat to cold, hard to soft, dense to rare, gross to fine, and so forth, and since these degrees are known by sensual and ocular experience, while degrees of height, that is, discrete degrees are not, therefore the latter shall be treated of especially in this Part, for without a knowledge of these [discrete] degrees, causes cannot be seen. Indeed it is known that end, cause and effect follow in order like first, middle and last. It is also known among other things that the end produces the cause, and through the cause, the effect so that the end may exist. And yet to know these things and not see them in their applications to existing things is merely to know abstractions which remain in the mind only so long as analytical ideas derived from metaphysical ones are in the thought. Hence it is that, although end, cause and effect proceed through discrete degrees, yet little if anything is known in the world about these degrees. For a mere knowledge of abstractions is like an airy something which flies away. But if abstractions are applied to such things as are in the world, they are like what is seen with the eyes on earth, and remain in the memory.