Here’s how EM Forster describes the Caves of Marabar:
 
The caves are readily described. A tunnel eight feet long, five feet high, three feet wide, leads to a circular chamber about twenty feet in diameter. This arrangement occurs again and again throughout the group of hills, and this is all, this is a Marabar Cave. Having seen one such cave, having seen two, having seen three, four, fourteen, twenty-four, the visitor returns to Chandrapore uncertain whether he has had an interesting experience or a dull one or any experience at all. He finds it difficult to discuss the caves, or to keep them apart in his mind, for the pattern never varies, and no carving, not even a bees'-nest or a bat distinguishes one from another. Nothing, nothing attaches to them, and their reputation-for they have one-does not depend upon human speech. It is as if the surrounding plain or the passing birds have taken upon themselves to exclaim "extraordinary," and the word has taken root in the air, and been inhaled by mankind.
 
This ‘extraordinary’ quality does not lend itself to pat explanations of its meaning. What this quality of ‘nothingness’ forces visitors to do is confront themselves - there’s no message to read into the caves, so their visitors’ gaze is forced inwards. And in the case of the English, the mirror that is held up to their souls reveals that all the structures they have erected in the name of ‘civilization’ are utterly illusory.
Yet this isn’t an excuse to leap to the conclusion that the world has no meaning. Mrs Moore, the sturdy English matriarch, is spiritually troubled by her Marabar experience, yet she gradually metamorphoses into an embodiment of the potentiality for human love. Her burial, not in a marked grave plot, but in the vastness of the sea, emphasizes the fact that by the end of the novel she is viewed as a force of nature, something the local Indians had already picked up on when they began to chant her name, as they would that of a deity.
So yes, the caves can symbolize emptiness, but only if you have nothing of substance inside you to bring out. It’s much more interesting to think of them as a catalyst, which is what the best symbols are, nodes that hold a web of related meanings in suspension. It’s true the effect of a visit can be explained in terms of culture, history and religion, but all three subjects are subtly subverted to create a unique psychological environment that is far more challenging and troublesome than a conventional interpretation.
One of the hallmarks of Forster’s achievement is that this rich spirituality doesn’t arrive courtesy of some cliched miasmal image of the Mystic East,  but takes shape within walls of solid rock - the very opposite of the hippy trippy land of stoned enchantment envisioned by the 1960’s counter-culture. It’s a wonderfully telling, if not altogether comprehensible juxtaposition.
So even a symbol as solid and elemental as the Caves of Marabar can retain an element of mystery, and, in the view of this reader at least, preserves some immunity from critical ingenuity, since every explanation of what they represent seems somehow incomplete and resistant to generalization. And like all the most successful symbols, they act as a sort of beacon within the text, a focus of attention to which the mind’s eye is repeatedly drawn. As such, they are also a structural component, drawing in different strands of meaning and characters around a distinct focal point.
In terms of our Meaning Line, I’d place them way over on the right if they are, as I’m suggesting, ‘scramblers’ of meaning.
Another famous symbol of this type is “Mistah Kurtz” in Joseph Conrad’s ‘Heart of Darkness’ which once again pitches a fragile rationality against incomprehensible forces born out of our subconscious. Throughout Marlow’s journey, he is confronted by signs that are more and more unrecognizable, because he and the others cannot impose meanings on them, or because the meanings they impose reflect their own anxieties more than anything else. Interestingly, Marlow as a storyteller has a frustrating reputation for fascinating people but never getting at the meaning of things; he keeps asking questions and suggesting mysteries rather than explaining and revealing - a bit like his creator. Kurtz gradually embodies for Marlow this problem of meaning. He never gets to be a flesh-and-blood person: he's a word, then a voice, then a set of ideals: always an abstraction.
 So, once again looking at our model of the schoolboy’s envelope, this reading expands Kurtz outwards from an individual human to a universal symbol of (among many other things) man’s problems with the inexpressible; Marlow continually hopes that Kurtz will enlighten him on the topic of meaning only to have his ambitions frustrated.
In some critical accounts, Kurtz has been characterized as an artist figure, one who converses with the deepest mysteries, and there’s something to be said for this - but he turns out to be the guide who has lost his way, leaving Marlow to simplistically conclude that “his soul was mad”.
Unlike, say, Macbeth, who is a flesh-and-blood character as well as a symbol of a specific quality, Kurtz has been deliberately  (and appropriately) represented as a void - making it easier for us to treat him as a vessel to fill with our own meanings, so again, he would occupy a position to the right on our Meaning Line. As such, both the Caves of Malabar and Kurtz would satisfy TS Eliot’s demand that art should “simplify current life into something rich and strange.” They are like icebergs, seven-eighths under the water with only the top bit poking out for all to see. As readers, we are left to wonder what lies beneath.
Slightly less rich and strange yet still inhabiting this general area of symbolism is the confectioner’s shop in Joanne Harris’s ‘Chocolat’. The novel as a whole reads like one that has been carefully planned and constructed, and this applies to the deliberateness of the symbolism too. Yet, perhaps in spite of itself, Vianne’s chocolate house doesn’t simply represent sensual indulgence. Again, like the schoolboy’s envelope, it can be attenuated outwards to embody a much less particular liberation from repression of all kinds (as if to emphasize this, the novel is set during Lent, the season of religiously-inspired self-denial).
As with the previous examples we’ve looked at, the symbol can accommodate a ‘top’, while making the reader sense that there are other forces at work which have far broader applications. This interplay between the specific and the symbolic is borne out by the writer herself, who says of her heroine;
 
Vianne is who you want her to be. You might see her as an archetype or a mythical figure (I prefer to see her as the lone gunslinger who blows into the town, has a showdown with the man in the black hat, then moves on relentless), but on another level she is a perfectly real person with real insecurities and a very human desire for love and acceptance. Her qualities too - kindness, love, tolerance - are very human. She is not supposed to be either a supernatural or a superhuman figure - everything she does exists on this everyday level, and she does nothing which could not be done perfectly well by anyone else.
 
So here’s an iceberg sticking further out of the water than Conrad’s or Forster’s; meaning isn’t being cut loose (as in the former) or turned in on itself (as in the latter), it’s being drawn out and having its scope extended in a more controlled way. Chocolate is still a symbol, but one that inhabits a position further to the right on our Meaning Line than the other two, since its significance is more easily encapsulated.
This won’t be all we’ve got to say about the Symbol - it’ll stalk the argument again when we deal with Myth in section (g). For now, though, what we need to take away from this bit of the discussion are the two organizational principles at work within the symbol’s meaning - the tension between form and meaning within the symbol itself, and the way the resulting significance radiates outwards. We should also note that in all three of the above examples, the imprecision of the symbol’s meaning exists to draw its readers into a dialogue with it, to open lines of communication with the text. This creative fuzziness at the heart of the symbol, this slightly imperfect match between form and meaning has given literature some of its most haunting and obsessively analysed images, but has also made it the most misinterpreted literary term by those hungry for resolution. And again, throwing forward to Part 2, we’ll see how irresolution can turn a seemingly unremarkable story into a classic.
 
Allegory
 
Well, the point of that, Dud, is that Genesis isn’t true in the literal sense. It’s an allergy. Genesis is an allergy of La Condition Humaine. It’s about the whole lot of the human race. Adam and Eve aren’t just Adam and Eve. They’re the whole human race personified.
 
Peter Cook & Dudley Moore, ‘Not Only But Also’, BBCTV, 1965
 
Our examination of the writer’s toolbox continues with yet another  variation on the theme of metaphor. If we represent the relationship between form and meaning as two cardboard circles, one placed over the other, the Symbol would have one circle slightly offset in relation to the other. The majority of their surface areas would overlap, but there’d be two crescent-shaped portions out on their own. And this would represent the ambiguity we noted just now.
The structure at work within the allegory would permit  no such imprecision; the two circles would overlap perfectly - form and meaning are fully integrated with one another. So allegory is a form of extended metaphor, in which objects, persons, and actions in a narrative, are equated with the meanings that lie outside the narrative itself. The underlying meaning has moral, social, religious, or political significance, and characters are often personifications of abstract ideas as charity, greed, or envy. Thus an allegory is a story with two meanings, a literal meaning and a metaphorical meaning. Simple. Compared with the Symbol, the way allegories work is an intellectual cakewalk.
The main point to note is that an allegory is more likely to have been deliberately constructed by the writer than a symbol, whose significance he may not understand. That deliberate element may imply that the writer’s aim is to educate his audience; by drawing down an abstraction, for example, and giving it physical form, he’ll make it easier for us to understand. Unlike the Symbolist, the drive is to simplify, and not to radiate meaning outwards. The allegorist believes he is taming meaning; the symbolist that he is setting it free. This distinction led the sainted CS Lewis to brilliantly observe that the allegorist starts with the real as the basis for his story, then moves out into fiction to illustrate it; the symbolist starts with real life, which he considers to be a fiction, and moves out into a realm of complex meaning which he considers is more real than real life.
Of course, once the allegorist has got his message across, that doesn’t stop him applying that message to as broad a swathe of humanity as he thinks feasible, but in the establishment of that message, A=B. So if I were to construct a story about a slug, it would more than likely concern laziness. I would then point at my audience and say, “see that slug? That’s you, that is!”.
 
Allegory can be as unsophisticated as that.