To understand the nature of the problem, we must first understand the world of Hume. He was born to the enlightenment era, a time of rapid growth in science and culture. For over a thousand years, the great debate was held, is truth found in the world of perception, or is it found in the realm of mathematical construct and metaphysical proposition. Plato and Aristotle held the latter view. It was this very belief that had been the glue that held society together, giving the supremacy to the Divine right of Kings and Papal Dogma. The problem was that this very premise collapsed when Galileo and Copernicus demonstrated the sun must be at the center of the solar system. Plato and Aristotle had been proven wrong. To this end, the question was asked, then what is truth?
To answer this question, Hume begins with the distinction between impressions and ideas. He argues that impressions are born of the world of experience, whereas an idea is the byproduct of that impression. To this end, he argues that a blind man has no idea of color because he has no memory of the occurrence, and hence no experience upon which to form the idea. It follows then, that an idea is an abstraction built on experience. This leads us to the question of shades of blue. Hume begins.
"We shall always find, that every idea which we examine is copied from a similar impression. Those who would assert, that this position is not universally true nor without exception, have only one, and at that an easy method of refuting it; by producing that idea, which, in their opinion, is not derived from this source."
The problem of the missing shade of blue arises because just two paragraphs later when Hume seems to provide just such an idea.
"There is, however, one contradictory phenomenon, which may prove, that it is not absolutely impossible for ideas to arise, independent of their correspondent impressions. I believe it will readily be allowed, that the several distinct ideas of colour, which enter by the eye, or those of sound, which are conveyed by the ear, are really different from each other; though, at the same time, resembling. Now if this be true of different colours, it must be no less so of the different shades of the same colour; and each shade produces a distinct idea, independent of the rest. For if this should be denied, it is possible, by the continual gradation of shades, to run a colour insensibly into what is most remote from it; and if you will not allow any of the means to be different, you cannot, without absurdity, deny the extremes to be the same. Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds, except one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain, that he will perceive a blank, where that shade is wanting, and will be sensible, that there is a greater distance in that place between the contiguous colours than in any other. Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can: And this may serve as a proof, that the simple ideas are not always, in every instance, derived from the correspondent impressions; though this instance is so singular, that it is scarcely worth our observing, and does not merit, that for it alone we should alter our general maxim."
While this exception to his concept of truth into human enquiry seems contradictory, it in many way demonstrates the entire principle of his thesis. There must be an exception to every rule. This leads us to the very heart of Hume's arguments. This is the problem of causality. Rationality is built on the foundation of cause and effect. That when in the world of experience we see A and B linked to one another, we can say that A has led to B. For example we can say that in the morning the sun rises. On every day we have lived, in the morning the sun has risen, so it reasonable to say that tomorrow, in the morning the sun will rise. However, is it the condition of morning that causes the sun to rise? Suppose we were to wake on the morning, this time on another world, like Pluto, very far away from the sun? Our experience in that case would be very different. Hence our very understanding of truth is contradictory in nature. This has led us to understand that when we say that A has caused B to occur, we say this on the basis of probability. On this assumption, we can say with no uncertainty that the sun will rise in the morning, however we do so in the knowledge that this is based in probability, and not a steadfast rule in itself. The exception of course is rare, however one day the sun will explode in a supernova, and there will be no morning. That is as certain as the sun will rise again.
