Isis Unveiled is organised as an attack on two fronts, and its two volumes are titled accordingly. The first is directed at science, arguing that the materialism of the period had excluded a range of real phenomena by definition rather than by investigation. The second is directed at theology, arguing that Christianity had inherited and then disowned an older wisdom, and reading comparative religion as evidence that all traditions descend from one source.
The method throughout is accumulation. Blavatsky piles up citations from Egyptology, Indology, classical authors, church fathers, mesmerism and contemporary science reporting, and the effect is of an argument made by sheer weight of material rather than by demonstration.
That method is what left the book open to the charge treated in the other article on this page. It is also why the book was influential: it made a case that the esoteric was a subject with a literature, at a moment when educated readers were being told it was not.
The doctrinal content that Theosophy became famous for is largely absent here. The root races, the septenary constitution and the Masters as a structured hierarchy arrive with The Secret Doctrine eleven years later.