The Pythagorean idea of the world was that natural numbers were the answer to the different secrets of humans and matter. They thought that everything was made up of numbers, the reason for what anything was could only be figured out in numbers (Early Concepts of Number and Number Mysticism).
For the numbers from 1 to 10 each number has a special property. The number 1 is a monad and stands for unity because it is the cause of the other numbers and in number mysticism is the number of reason. The number 2 is called a dyad for diversity or opinion and is mystically the first female number (all the even numbers are called female in many early cultures). The number 3 is a triad and means harmony because it adds up unity (number 1) plus diversity (number 2) and is considered the first male number (all the odd numbers are male in many early cultures). Number 4 is the number for justice or the squaring of accounts (number 2 squared). Number 5 is for marriage because the number 2 (first female) plus the number 3 (first male) equals the number 5. Number 6 is for creation because the number 2 (first female) plus the number 3 (first male) plus 1 more monad equals 6. The Number 10 is for the Universe because as tetractys it is the sum of all possible dimensions in geometry.