This article traces essential sources behind the Western reception of Sanskrit terminology on the concept of subtle anatomy, focusing on the late nineteenth-century when the Theosophical Society and its forefront, Helena Petrovna Blavatsky, first presented it to a Western audience. A doctrinal change took place around 1880–81 in an interaction between American, European, and Indian Theosophists, distinguishing Blavatsky’s major works Isis Unveiled (1877) and The Secret Doctrine (1888). The subject of how and why the first doctrine of three human principles (body, soul, and spirit) developed into her later version including seven human principles is carefully examined. A new hypothesis on why the number seven became the backbone of Blavatsky’s entire cosmology is also presented. According to this, the seven-fold subtle anatomy was there since the grounding of the Theosophical Society (1875) and was rooted in specific numerological, mathematical, and geometric speculations which Blavatsky shared with several other contemporary authors. The article explores Blavatsky’s interpretation of some related arithmological themes in nineteenth-century American literature such as the Pythagorean tetraktys, “the tetrad,” “the pyramid,” “the cube,” and “the hexagram.”