{xcv} There is more difficulty in comprehending how he arrived at the first term of the series, which is nowhere else mentioned, and has no reference to any other part of his system. Nor indeed does the relation of shadows to objects correspond to the relation of numbers to ideas. Probably Plato has been led by the love of analogy (Timaeus, p. 32 B) to make four terms instead of three, although the objects perceived in both divisions of the lower sphere are equally objects of sense. He is also preparing the way, as his manner is, for the shadows of images at the beginning of the seventh book, and the imitation of an imitation in the tenth. The line may be regarded as reaching from unity to infinity, and is divided into two unequal parts, and subdivided into two more; each lower sphere is the multiplication of the preceding. Of the four faculties, faith in the lower division has an intermediate position (cp. for the use of the word faith or belief, [Greek: pi/stis], Timaeus, 29 C, 37 B), contrasting equally with the vagueness of the perception of shadows ([Greek: ei)kasi/a]) and the higher certainty of understanding ([Greek: dia/noia]) and reason ([Greek: nou=s]). The difference between understanding and mind or reason ([Greek: nou=s]) is analogous to the difference between acquiring knowledge in the parts and the contemplation of the whole. True knowledge is a whole, and is at rest; consistency and universality are the tests of truth. To this self-evidencing knowledge of the whole the faculty of mind is supposed to correspond. But there is a knowledge of the understanding which is incomplete and in motion always, because unable to rest in the subordinate ideas. Those ideas are called both images and hypotheses--images because they are clothed in sense, hypotheses because they are assumptions only, until they are brought into connexion with the idea of good. The general meaning of the passage 508-511, so far as the thought contained in it admits of being translated into the terms of modern philosophy, may be described or explained as follows:--There is a truth, one and self-existent, to which by the help of a ladder let down from above, the human intelligence may ascend. This unity is like the sun in the heavens, the light by which all things are seen, the being by which they are created and sustained. It is the _idea_ of good. And the steps of the ladder leading up to this highest or universal existence are the mathematical {xcvi} sciences, which also contain in themselves an element of the universal. These, too, we see in a new manner when we connect them with the idea of good. They then cease to be hypotheses or pictures, and become essential parts of a higher truth which is at once their first principle and their final cause.