In his work Phaedo, Plato develops this idea in a consistent way. If we ask what the cause of a thing is, we end up with its essence—the Greek word is "eidos," which can be variously translated as form or idea, although Aristotle interprets it as "species," which is obviously preferable from a materialist standpoint. To go back to our dinner-plate. What makes it round? or—to use Platonic language—What is the cause of its roundness? One might answer, that it was caused by a potter rotating a lump of clay on a wheel and moulding it with his hand. But for Plato, the plate, like all other crude material objects, is merely an imperfect manifestation of the Idea, which, put in plain language, is God.
Plato’s theory of knowledge, which Aristotle says is different from that of Socrates, was based on the idea that the object of knowledge must be permanent, eternal, and since nothing under the sun is permanent, we must seek stable knowledge outside this fleeting and deceitful world of material things. When Diogenes ridiculed the theory of Ideas, by saying he could see the cup, but not "cupness," Plato retorted that that was because he had eyes to see, but no intellect. And it is true that merely to base oneself on sense-perception is not enough. It is necessary to go from the particular to the universal. The fundamental flaw here is to think that the generalisations of the intellect can stand on their own, divorced from, and counterposed to, the material world from which, ultimately, they are derived.
Marx and Engels in The Holy Family explained: in the philosophy of Idealism, the real relations between thought and being are stood on their head, "for the absolute idealist, in order to be an absolute idealist, must necessarily constantly go through the sophistical process of first transforming the world outside himself into an appearance, a mere fancy of his brain, and afterwards declaring this fantasy to be what it really is, i.e., a mere fantasy, so as finally to be able to proclaim his sole, exclusive existence, which is no longer disturbed even by the semblance of an external world." (MECW, Vol. 4, p. 140.)
The sophistical trick whereby this is done was wittily explained in the same work:
"If from real apples, pears, strawberries and almonds I form the general idea ‘Fruit,’ if I go further and imagine that my abstract idea ‘Fruit,’ derived from real fruit, is an entity existing outside me, is indeed the true essence of the pear, the apple, etc., then—in the language of speculative philosophy—I am declaring that ‘Fruit’ is the ‘Substance’ of the pear, the apple, the almond, etc. I am saying, therefore, that to be a pear is not essential to the pear, that to be an apple is not essential to the apple; that what is essential to these things is not their real existence, perceptible to the senses, but the essence that I have abstracted from them and then foisted on them, the essence of my idea—’Fruit.’ I therefore declare apples, pears, almonds, etc., to be mere forms of existence, modi, of ‘Fruit.’ My finite understanding supported by my senses does of course distinguish an apple from a pear and a pear from an almond, but my speculative reason declares these sensuous differences inessential and irrelevant. It sees in the apple the same as in the pear, and in the pear the same as in the almond, namely ‘Fruit.’ Particular real fruits are no more than semblances whose true essence is ‘the substance’—’Fruit.’ (Ibid, pp. 57-8.)
Far from advancing the cause of human understanding, the idealist method does not take us a single step forward. Only a study of the real, that is to say, material world, can deepen our understanding of nature and our place in it. By directing men’s eyes away from "crude" material things towards the realm of so-called "pure" abstraction, the idealists played havoc with the development of science for centuries. "By this method one attains no particular wealth of definition. The mineralogist whose science was limited to the statement that all minerals are really ‘the Mineral’ would be a mineralogist only in his imagination. For every mineral the speculative mineralogist says ‘the Mineral,’ and his science is reduced to repeating this word as many times as there are real minerals." (Ibid.)
As opposed to the earlier Greek philosophers, who were generally materialists, and set out from a study of nature, Plato consciously turned his back on the world of the senses. Not experiment and observation, but only pure deduction and mathematics was the road to truth. Above the entrance of his Academy in Athens he placed the inscription: "Let no man destitute of geometry enter my doors." Plato encouraged his students, for example, to study the stars, not as they are, but as they ought to be. Following in the footsteps of the Pythagoreans, he alleged that the planets showed their divine nature by their eternally unchanging orbits, the perfect regularity of their circular motion being an expression of the harmony of the universe. This cosmology, together with that of Aristotle, his great successor, held back the development of astronomy for 2,000 years. It represented a retreat from science to Pythagorean mysticism. Thus, in an Alexandrian hand-book on astronomy written by Geminus, we read:
"There underlies the whole science of astronomy,…the assumption that the sun and the moon and the five planets move at even speeds in perfect circles in an opposite direction to the cosmos. It was the Pythagoreans, the first to approach these questions, who laid down the hypothesis of a circular and uniform motion for the sun, moon, and planets. Their view was that, in regard of divine and eternal beings, a supposition of such disorder as that these bodies should move now more quickly and now more slowly, or should even stop, as in what are called the stations of the planets, is inadmissible. Even in the human sphere such irregularity is incompatible with the orderly procedure of a gentleman. And even if the crude necessities of life often impose upon men occasions of haste or loitering, it is not to be supposed that such occasions inhere in the incorruptible nature of the stars. For this reason they defined their problem as the explanation of the phenomena on the hypothesis of circular and uniform motion." (Farrington, Greek Science, pp. 95-6.)
Kepler discovered that the planets moved, not in circles, but in ellipses. Even this was not completely true, as Newton later showed. The ellipses are not perfect, either. But for the previous two millennia, the idealist picture of the universe held the force of an unchallengeable dogma. For much of that time it was backed by the formidable power of the Church.
It is significant that the ideas of Plato were known in the Middle Ages through only one work, the Timaeus, his worst book. This represents a complete counter-revolution in philosophy. From Thales on, Greek philosophy was characterised by an attempt to explain the world in natural terms, without recourse to the gods or any supernatural phenomena. The Timaeus is not a work of philosophy but a religious tract. Here we see a revival of "all the old crap," as Marx once put it. It is, in effect, the revival of the old creation myth. The world was created by a Supreme Craftsman. Matter consists of triangles because solids are bounded by planes, and planes can be resolved into triangles. The world is spherical and moves in circles because the circle is the most perfect form. Men who live badly are reborn as women in the next reincarnation, and so on and so forth.
In a passage strikingly similar to some of the statements of the present-day defenders of the "big bang," Plato writes about the "beginning of time":
"Time, then, and the heaven came into being at the same instant in order that, having been created together, if ever there was to be a dissolution of them, they might be dissolved together. It was framed after the pattern of the eternal nature, that it might resemble this as far as was possible; for the pattern exists from eternity, and the created heaven has been, and is, and will be, in all time. Such was the mind and thought of God in the creation of time." (The Dialogues of Plato, Jowett’s edition, Vol. 3, Timaeus, p. 242.) No wonder the Christian Church welcomed this with open arms!
Despite its dialectical side, the Platonic philosophy is essentially a conservative one, reflecting the world outlook of an aristocratic elite, who felt, correctly, that their world was crumbling about them. The urge to turn one’s back on reality, to deny the evidence of one’s senses, to cling to some kind of stability in the midst of turbulence and upheaval, to deny change, all this clearly corresponded to a powerful psychological and moral need.