The Fundamental Principle of Digits of a Number is a new mathematical idea for which the author received a copyright from the United States Library of Congress. Two related concepts make it easy to understand and apply the principle. The first concept is that a permutation of digits of a given number is an arrangement of the digits of the given number in any order such that the numerical quantity, which results from the arrangement of the digits of the given number, has the same digits and the same number of digits as the given number. The second concept is that the difference between two permutations of digits of a given number is governed by a mathematical law which guarantees that the difference is divisible by 9. One day, the number 12 suddenly appeared on the author's inner eye. It turned around and formed the number 21. The two numbers subtracted, and number 9 appeared. Then the three numbers disappeared from the author's inner eye. The motion of the numbers by their own power, as if they were birds in the sky, prompted Chibamba Mulenga to investigate this event with digits of other numbers, leading to his discovery of this mathematical principle.