The most contested move in Grant's work is methodological: he treated imaginative fiction as evidence. In the Typhonian trilogies, H. P. Lovecraft's invented mythos appears alongside Crowley's received texts and Spare's automatic drawings as testimony to the same submerged reality, on the argument that the deep imagination does not invent but receives, and that a horror writer's dreams may be soundings of the same nightside the magician enters deliberately. Lovecraft the materialist would have been appalled, as Grant knew and rather relished.
The doctrine has a machinery: Nightside of Eden maps the tunnels of Set, the reverse of the kabbalistic Tree, assigning to each dark pathway its entities and sigils, and proposing that genuine initiation must traverse and integrate these averse states rather than pretend the Tree has only a daylight face. His New Isis Lodge, operating in London from 1955 to 1962, worked rites built on these premises, incorporating Spare's sigil methods and trance work.
Critics inside and outside the Thelemic world have called the system unfalsifiable, the scholarship loose and the prose oracular, and Grant's claimed authority over the OTO was disputed for decades. The influence is not disputed: chaos magic's free use of fictional pantheons, and the whole later conversation about belief as a tool rather than a conclusion, begin in Grant's deadpan refusal to distinguish the imagined from the encountered.