Northrop Frye says

Suppose you're shipwrecked on an uninhabited island in the South Seas. The first thing you do is to take a long look at the world around you, a world of sky and sea and earth and stars and trees and hills. You see this world as objective, as something set over against you and not yourself or related to you in any way. And you notice two things about this objective world. In the first place, it doesn't have any conversation. It's full of animals and plants and insects going on with their own business, but there's nothing that responds to you: it has no morals and no intelligence, or at least none that you can grasp. It may have a shape and a meaning, but it doesn't seem to be a human shape or a human meaning. Even if there's enough to eat and no dangerous animals, you feel lonely and frightened and unwanted in such a world.

In the second place, you find that looking at the world, as something set over against you, splits your mind in two. You have an intellect that feels curious about it and wants to study it, and you have feelings or emotions that see it as beautiful or austere or terrible. You know that both these attitudes have some reality, at least for you. If the ship you were wrecked in was a Western ship, you'd probably feel that your intellect tells you more about what's really there in the outer world, and that your emotions tell you more about what's going on inside you. If your background were Oriental, you'd be more likely to reverse this and say that the beauty or terror was what was really there, and that your instinct to count and classify and measure and pull to pieces was what was inside your mind. But whether your point of view is Western or Eastern, intellect and emotion never get together in your mind as long as you're simply looking at the world. They alternate, and keep you divided between them.

The language you use on this level of the mind is the language of consciousness or awareness. It's largely a language of nouns and adjectives. You have to have names for things, and you need qualities like 'wet' or 'green' or 'beautiful' to describe how things seem to you. This is the speculative or contemplative position of the mind, the position in which the arts and sciences begin, although they don't stay there very long. The sciences begin by accepting the facts and the evidence about an outside world without trying to alter them. Science proceeds by accurate measurement and description, and follows the demands of the reason rather than the emotions. What it deals with is there, whether we like it or not. The emotions are unreasonable: for them it's what they like and don't like that comes first. We'd be naturally inclined to think that the arts follow the path of emotion, in contrast to the sciences. Up to a point they do, but there's a complicating factor.

That complicating factor is the contrast between 'I like this' and 'I don't like this'. In this Robinson Crusoe life I've assigned you, you may have moods of complete peacefulness and joy, moods when you accept your island and everything around you. You wouldn't have such moods very often, and when you had them, they'd be moods of identification, when you felt that the island was a part of you and you a part of it. That is not the feeling of consciousness or awareness, where you feel split off from everything that's not your perceiving self. Your habitual state of mind is the feeling of separation which goes with being conscious, and the feeling 'this is not a part of me' soon becomes 'this is not what I want'....

So you soon realize that there's a difference between the world you're living in and the world you want to live in. The world you want to live in is a human world, not an objective one: it's not an environment but a home; it's not the world you see but the world you build out of what you see. You go to work to build a shelter or plant a garden, and as soon as you start to work you've moved into a different level of human life. You're not separating only yourself from nature now, but constructing a human world and separating it from the rest of the world. Your intellect and emotions are now both engaged in the same activity, so there's no longer any real distinction between them. As soon as you plant a garden or a crop, you develop the conception of a 'weed', the plant you don't want in there. But you can't say that 'weed' is either an intellectual or an emotional conception, because it's both at once. Further, you go to work because you feel you have to, and because you want something at the end of the work. That means that the important categories of your life are no longer the subject and the object, the watcher and the things being watched: the important categories are what you have to do and what you want to do — in other words, necessity and freedom.

One person by himself is not a complete human being, so I'll provide you with another shipwrecked refugee of the opposite sex and an eventual family. Now you're a member of a human society. This human society after a while will transform the island into something with a human shape. What that human shape is, is revealed in the shape of the work you do: the buildings, such as they are, the paths through the woods, the planted crops fenced off against whatever animals want to eat them. These things, these rudiments of city, highway, garden and farm, are the human form of nature, or the form of human nature, whichever you like. This is the area of the applied arts and sciences, and it appears in our society as engineering and agriculture and medicine and architecture. In this area we can never say clearly where the art stops and the science begins, or vice versa.

The language you use on this level is the language of practical sense, a language of verbs or words of action and movement. The practical world, however, is a world where actions speak louder than words. In some ways it's a higher level of existence than the speculative level, because it's doing something about the world instead of just looking at it, but in itself it's a much more primitive level. It's the process of adapting to the environment, or rather of transforming the environment in the interests of one species, that goes on among animals and plants as well as human beings. The animals have a good many of our practical skills: some insects make pretty fair architects, and beavers know quite a lot about engineering. In this island, probably, and certainly if you were alone, you'd have about the ranking of a second-rate animal. What makes our practical life really human is a third level of the mind, a level where consciousness and practical skill come together.

This third level is a vision or model in your mind of what you want to construct. There's that word 'want' again. The actions of man are prompted by desire, and some of these desires are needs, like food and warmth and shelter. One of these needs is sexual, the desire to reproduce and bring more human beings into existence. But there's also a desire to bring a social human form into existence: the form of cities and gardens and farms that we call civilization. Many animals and insects have this social form too, but man knows that he has it: he can compare what he does with what he can imagine being done. So we begin to see where the imagination belongs in the scheme of human affairs. It's the power of constructing possible models of human experience. In the world of the imagination, anything goes that's imaginatively possible, but nothing really happens. If it did happen, it would move out of the world of imagination into the world of action.

We have three levels of the mind now, and a language for each of them, which in English-speaking societies means an English for each of them. There's the level of consciousness and awareness, where the most important thing is the difference between me and everything else. The English of this level is the English of ordinary conversation, which is mostly monologue, as you'll soon realize if you do a bit of eavesdropping, or listening to yourself. We can call it the language of self-expression. Then there's the level of social participation, the working or technological language of teachers and preachers and politicians and advertisers and lawyers and journalists and scientists. We've already called this the language of practical sense. Then there's the level of imagination, which produces the literary language of poems and plays and novels. They're not really different languages, of course, but three different reasons for using words.

On this basis, perhaps, we can distinguish the arts from the sciences. Science begins with the world we have to live in, accepting its data and trying to explain its laws. From there, it moves towards the imagination: it becomes a mental construct, a model of a possible way of interpreting experience. The further it goes in this direction, the more it tends to speak the language of mathematics, which is really one of the languages of the imagination, along with literature and music. Art, on the other hand, begins with the world we construct, not with the world we see. It starts with the imagination, and then works towards ordinary experience: that is, it tries to make itself as convincing and recognizable as it can. You can see why we tend to think of the sciences as intellectual and the arts as emotional: one starts with the world as it is, the other with the world we want to have. Up to a point it is true that science gives an intellectual view of reality, and that the arts try to make the emotions as precise and disciplined as sciences do the intellect. But of course it's nonsense to think of the scientist as a cold unemotional reasoner and the artist as somebody who's in a perpetual emotional tizzy. You can't distinguish the arts from the sciences by the mental processes the people in them use: they both operate on a mixture of hunch and common sense. A highly developed science and a highly developed art are very close together, psychologically and otherwise.

Still, the fact that they start from opposite ends, even if they do meet in the middle, makes for one important difference between them. Science learns more and more about the world as it goes on: it evolves and improves. A physicist today knows more physics than Newton did, even if he's not as great a scientist. But literature begins with the possible model of experience, and what it produces is the literary model we call the classic. Literature doesn't evolve or improve or progress. We may have dramatists in the future who will write plays as good as King Lear, though they'll be very different ones, but drama as a whole will never get better than King Lear. King Lear is it, as far as drama is concerned; so is Oedipus Rex, written two thousand years earlier than that, and both will be models of dramatic writing as long as the human race endures. Social conditions may improve: most of us would rather live in nineteenth-century United States than in thirteenth-century Italy, and for most of us Whitman's celebration of democracy makes a lot more sense than Dante's Inferno. But it doesn't follow that Whitman is a better poet than Dante: literature won't line up with that kind of improvement. So we find that everything that does improve, including science, leaves the literary artist out in the cold. Writers don't seem to benefit much by the advance of science, although they thrive on superstitions of all kinds....

Literature belongs to the world man constructs, not to the world he sees; to his home, not his environment. Literature's world is a concrete human world of immediate experience. The poet uses images and objects and sensations much more than he uses abstract ideas; the novelist is concerned with telling stories, not with working out arguments. The world of literature is human in shape, a world where the sun rises in the east and sets in the west over the edge of a flat earth in three dimensions, where the primary realities are not atoms or electrons but bodies, and the primary forces not energy or gravitation but love and death and passion and joy. It's not surprising if writers are often rather simple people, not always what we think of as intellectuals, and certainly not always any freer of silliness or perversity than anyone else. What concerns us is what they produce, not what they are, and poetry, according to Milton, who ought to have known, is 'more simple, sensuous and passionate' than philosophy or science....

At the level of ordinary consciousness the individual man is the centre of everything, surrounded on all sides by what he isn't. At the level of practical sense, or civilization, there's a human circumference, a little cultivated world with a human shape, fenced off from the jungle and inside the sea and the sky. But in the imagination anything goes that can be imagined, and the limit of the imagination is a totally human world. Here we recapture, in full consciousness, that original lost sense of identity with our surroundings, where there is nothing outside the mind of man, or something identical with the mind of man. ...

Outside literature, the main motive for writing is to describe this world. But literature itself uses language in a way which associates our minds with it. As soon as you use associative language, you begin using figures of speech. If you say this talk is dry and dull, you're using figures associating it with bread and breadknives. There are two main kinds of association, analogy and identity, two things that are like each other and two things that are each other.... One produces the figure of speech called the simile; the other produces the figure called metaphor.

In descriptive writing you have to be careful of associative language. You'll find that analogy, or likeness to something else, is very tricky to handle in description, because the differences are as important as the resemblances. As for metaphor, where you're really saying 'this is that', you're turning your back on logic and reason completely, because logically two things can never be the same thing and still remain two things. The poet, however, uses these two crude, primitive, archaic forms of thought in the most uninhibited way, because his job is not to describe nature, but to show you a world completely absorbed and possessed by the human mind. So he produces what Baudelaire called a 'suggestive magic including at the same time object and subject, the world outside the artist and the artist himself.' The motive for metaphor, according to Wallace Stevens, is a desire to associate, and finally to identify, the human mind with what goes on outside it, because the only genuine joy you can have is in those rare moments when you feel that although we may know in part, as Paul says, we are also a part of what we know.