Seven days of the week, seven seas, seven primary colors, seven wonders of the world, seven notes of the musical scale, the seven ages of man, seven deadly sins, seven levels of hell, seven categories for absolute judgment, the seven objects in the span of attention. Think 7-Up, Seven Dwarfs, The Magnificent Seven, seven-a-side. The sum of the opposite numbers in a pair of dice. Ask anyone to give you any number between one and ten and most will choose seven.
As I was going to St Ives
I met a man with seven wives
Each wife had seven sacks
Each sack had seven cats
Each cat had seven kits
Kits, cats, sacks and wives,
How many were going to St Ives?
7 is indeed a beautiful number… and might be uncanny but it is not magical, as cognitive psychologist George A. Miller explained in his 1956 paper The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information.
The above verse, by the way, was found on the Rhind papyrus written by Ahmes in Egypt about 1650 BC. And the answer is 1. Which also happens to be the number of thoughts you can have at any given single moment.
The Rhind papyrus