History of Western Philosophy - Bertrand Russell [75]
This experience, I believe, is necessary to good creative work, but it is not sufficient; indeed the subjective certainty that it brings with it may be fatally misleading. William James describes a man who got the experience from laughing-gas; whenever he was under its influence, he knew the secret of the universe, but when he came to, he had forgotten it. At last, with immense effort, he wrote down the secret before the vision had faded. When completely recovered, he rushed to see what he had written. It was: 'A smell of petroleum prevails throughout.' What seems like sudden insight may be misleading, and must be tested soberly, when the divine intoxication has passed.
Plato's vision, which he completely trusted at the time when he wrote the Republic, needs ultimately the help of a parable, the parable of the cave, in order to convey its nature to the reader. But it is led up to by various preliminary discussions, designed to make the reader see the necessity of the world of ideas.
First, the world of the intellect is distinguished from the world of the senses; then intellect and sense-perception are in turn each divided into two kinds. The two kinds of sense-perception need not concern us; the two kinds of intellect are called, respectively, 'reason' and 'understanding'. Of these, reason is the higher kind; it is concerned with pure ideas, and its method is dialectic. Understanding is the kind of intellect that is used in mathematics; it is inferior to reason in that it uses hypotheses which it cannot test. In geometry, for example, we say: 'Let ABC be a rectilinear triangle.' It is against the rules to ask whether ABC really is a rectilinear triangle, although, if it is a figure that we have drawn, we may be sure that it is not, because we can't draw absolutely straight lines. Accordingly, mathematics can never tell us what is, but only what would be if…. There are no straight lines in the sensible world; therefore, if mathematics is to have more than hypothetical truth, we must find evidence for the existence of super-sensible straight lines in a super-sensible world. This cannot be done by the understanding, but according to Plato it can be done by reason, which shows that there is a rectilinear triangle in heaven, of which geometrical propositions can be affirmed categorically, not hypothetically.
There is, at this point, a difficulty which did not escape Plato's notice, and was evident to modern idealistic philosophers. We saw that God made only one bed, and it would be natural to suppose that he made only one straight line. But if there is a heavenly triangle, he must have made at least three straight lines. The objects of geometry, though ideal, must exist in many examples; we need the possibility of two intersecting circles, and so on. This suggests that geometry, on Plato's theory, should not be capable of ultimate truth, but should be condemned as part of the study of appearance. We will, however, ignore this point, as to which Plato's answer is somewhat obscure.
Plato seeks to explain the difference between clear intellectual vision and the confused vision of sense-perception by an analogy from the sense of sight. Sight, he says, differs from the other senses, since it requires not only the eye and the object, but also light. We see clearly objects on which the sun shines: in twilight we see confusedly, and in pitch-darkness not at all. Now the world of ideas is what we see when the object is illumined by the sun, while the world of passing things is a confused twilight world. The eye is compared to the soul, and the sun, as the source of light, to truth or goodness.
The soul is like an eye: when resting upon that on which truth and being shine, the soul perceives and understands, and is radiant with intelligence; but when turned towards the twilight of becoming and perishing, then she has opinion only, and goes