The world soul is an explanation of the cosmos; a physical wave structure described by Plato in the form of seven numbers: 1, 2, 3, 4, 8, 9, and 27. The reason that there are seven is that there are seven perspectives required to describe the structure of the compound forms that make up the cosmos. That belief stems from the ancient Greek philosophy of how the cosmos operates. They believed that the universe was a system of numbers, interacting to from a balance between opposite poles.
The perspectives needed are:
1) One – signifying
unity and
perfection.
2) Two – signifying
duality. In opposition to
unity.
3) Numbers to
infinity, arrived at by the interaction between One and Two (which can be mathematically simulated by adding them together).
4) Points – can be represented by numbers to describe points in space.
5) Lines – can be created by connecting two points.
6) Shapes – Can be created by connecting lines together (as in connecting three points to form a triangle).
7) Solids – Can be created by connecting two-dimensional shapes. Such solid figures include pyramids, cubes, etc.
Solids can be compounded then to form anything in the universe, thus completing the circle (as the compound of all compounds is One – unified perfection).
Plato arrived at the numbers for his world soul by exploring the relationships which exist between the intervals uncovered by the Pythagorean hammers.
A deeper understanding can be found in the following books:
Capra, Fritjof _The Tao of Physics: An Exploration of the Parallels Between Modern Physics and Eastern Mysticism_. Massechusets: Shambhalla Publications, 1999
James, Jamie _The Music of the Spheres: Music, Science, and the Natural Order of the Universe_. New York: Grove Press, 1993