Seven

The Curious Symbolism of the Number Seven in Literature and Myth
After the number three, seven is perhaps the number that has the most religious significance around the world. There are seven deadly sins, seven gifts of the Holy Spirit, seven virtues, seven arts and sciences (comprising the quadrivium and trivium), and seven ages of man. Of course, there are seven days in the week, too, so the number seven has (literally) everyday significance. For a closer look at the strange attraction of the number seven in different cultures and the symbolism of sevens in literature, religion, and myth, 

The number seven in classical times
Sumerian and Akkadian scripture, which predates even Judaism, mentions seven demons that are symbolized in the well-known constellation, the Pleiades (commonly known as the Seven Sisters). In Judaism, there are seven branches of the Menorah. In ancient times, it was believed there were seven planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and the Moon. A. E. Housman, himself a classical scholar as well as a poet, writes a quatrain that references this old belief (before Uranus and Neptune were added to the planetary total):

Here are the planets and the stars. 
And all their fiery trains. Content you with the mimic heaven, And on the earth remain. Of course, calling the Moon a ‘planet’ was a bit of a stretch even in classical times, and it demonstrates the human desire to make a certain pattern ‘fit’ the number seven at all costs. Such a desire also underpins another scientific idea, the number of colors in the color spectrum. How many are there? Everyone knows there are seven: red, orange, yellow, green, blue, indigo, and violet. Right? Well, not quite. The only reason ‘indigo’ — a color that overlaps a fair bit with both ‘blue’ and ‘violet’ in the list — is in there is that Isaac Newton, who discovered that constituent ‘rainbow’ colors make up white light, wanted the number of colors to equal seven because Newton, for all his scientific empiricism, entertained many now-discredited views surrounding alchemy, mysticism, and the occult.

Seven Wonders of the World
Famously, too, there were seven wonders of the ancient world, only one of which remains today: the Great Pyramid of Giza (pictured below right). The others were the Hanging Gardens of Babylon, the Mausoleum at Halicarnassus, the Pharos (lighthouse) at Alexandria, the Temple of Artemis, the Statue of Zeus, and the Colossus of Rhodes.
However, despite the name, the list in its final form was not decided upon until much later, in the Renaissance, by which time all but one of these feats of architecture had already long been destroyed—and one of them, the Hanging Gardens of Babylon, may never have existed at all (and if they did, they may not actually have been in Babylon).

It was a poet, Antipater of Sidon, who first drew up a list of seven wonders in around 100 BC, but Antipater included Babylon twice (he counted the walls of the city and its gardens as two separate wonders, as did later poets such as Martial) and didn’t include the Pharos of Alexandria. It was a later writer, Gregory of Tours, who introduced this to replace the Walls of Babylon, although a fair bit of Gregory’s list was somewhat different from Antipater’s, including as it did the theater of Herakleia and even Noah’s Ark. Once again, though, his list named seven wonders.

Seven-symbolism in Christianity
Although the notorious number of the beast is found in the Book of Revelation, seven is the number that figures most frequently throughout that final book of the Bible: seven heads of the dragon (Revelation 12:3), seven churches, and seven horns and eyes of the lamb of God (5:6). But throughout the Old Testament, things keep coming in sevens: the seven spirits resting on Jesse’s rod, the seven heavens containing the different orders of angels, the seven years Solomon spent building his temple, and so on. And indeed, throughout Christianity, even outside of the Bible, the number seven looms large: as we remarked at the beginning of this article, there are seven deadly sins (pride, avarice or greed, gluttony, sloth, wrath, lust, and envy). Curiously, there could have ended up being eight deadly sins: the Desert Fathers (in particular, a fourth-century monk named Evagrius Ponticus), who identified a number of widespread evil thoughts that needed to be suppressed or vanquished, listed eight grave sins, which initially included things like acedia, dejection, and sadness (though this was specifically in regard to evil thoughts). It’s telling that the number seven was settled upon once again, although this was an improvement on Evagrius’ list, which had boasting and pride as separate, though clearly overlapping, sins. Meanwhile, although there are famously three theological virtues—Faith, Hope, and Charity—when these are added to the four cardinal virtues—Prudence, Temperance, Justice, and Fortitude—they form the fullness of the number seven.

Seven-symbol literature
The number seven has also been used by many writers over the centuries for its symbolism or even as a nice number to make up a series of titles: when C. S. Lewis wrote seven volumes in his Chronicles of Narnia fantasy series for children in the 1950s, he set a trend that has, in its way, been as influential as Tolkien’s three-part The Lord of the Rings, also from the 1950s, has been in establishing the ‘fantasy trilogy’ as a publishing phenomenon. Curiously, this happened because of a publisher's decision rather than Tolkien’s own intention: post-war paper shortages meant that the single-volume novel had to be published as three volumes, although in actual fact, The Lord of the Rings is divided into six, not three, ‘books’. So, J. K. Rowling published seven volumes in her bestselling Harry Potter series, while George R. R. Martin is set to complete his epic A Song of Ice and Fire (assuming he ever completes it) in seven novels (although this, too, is an example of a tendency to make things ‘fit’ the established pattern of seven: the third novel in the series, A Storm of Swords, was so long that it was published in two parts, which are, for all intents and purposes, individual novels). Of course, in classic works of literature, the number seven turns up in the stories themselves: seven dwarfs accompany Snow White, there are seven brides for seven brothers in the famous musical (based on a story, ‘The Sobbin’ Women’, by Stephen Vincent Benét, which is itself based on the biblical legend of the Sabine women), and when William Empson wrote his landmark study of ambiguity in poetry in 1930, one of the most influential works of literary criticism ever published, he listed seven types of ambiguity.

Why is seven such a symbolic number?
In 1956, George Miller demonstrated that most people can retain roughly seven items of information in their short-term memory. But as early as the sixteenth century, the Italian Jesuit theologian Robert Bellarmine argued that nobody can remember more than seven of anything to justify why his catechism omitted one of the Eight Scriptures.
This may help to explain why the number seven has had such powerful symbolism ever since ancient times: it seems to reach to the natural limit of our memory and thus embodies totality and fullness.