Annie Besant 
The Seven Principles of Man [EPUB ebook] 

Seven is one of the most important numbers in the teachings of theosophy, the wisdom-religion. It is a key by which are revealed and explained many of the mysteries of nature, for in theosophy it is taught that number and numbers underlie all the processes of creation. This numerical key of seven lies at the root of all evolution, whether physical or spiritual. It is therefore important both in its meanings and application.
There is nothing arbitrary about the use of seven as applied to the study of our composite constitution, as this number is found to be universal throughout the universe. We discover it everywhere. Some common facts are good instances of this, such as the seven layers of the human skin, the musical scale with its seven notes, and the seven colors which make up a ray of sunlight. Then there is the moon, which theosophy and science both show to have close relation to the generation of physical life on our planet. The moon is governed in its activities by the number seven. Note the recurrence of seven in the gestation period, the phases of the moon, with the week of seven days, etc. The inquirer has only to look observantly into the matter to find many other examples of the septenary in the world about him.