To kill a thing that turns its beholders to stone, the hero must first become unseeable. Sent to fetch the Gorgon's head, Perseus is equipped by the nymphs with winged sandals and a wallet, and "They had also the cap of Hades." He slings on the wallet, fits the sandals to his ankles, and "put the cap on his head." Wearing it, the record says, "he saw whom he pleased, but was not seen by" others — invisibility given as a plain gift, one item of a traveller's kit. When the deed is done and Medusa beheaded, the surviving Gorgons start from their sleep and pursue him, but "they could not see" him: "he was hidden by it." This is the motif Thompson filed as the magic cap.

The witness is The Library (a Greek mythography compiled in the first or second century CE), in the edition and translation of the folklorist J. G. Frazer. The cap belongs to Hades, the lord of the dead, and its power is exactly the power of that realm: it does not make its wearer strong or swift but simply subtracts him from sight, so that a man may walk among monsters and out again untouched. In the story it is one of three loaned wonders — cap, wallet, sandals — and when Perseus has finished he gives the wallet and the cap back to Hermes, who restores them to the nymphs; the borrowed invisibility is returned to the shelf it came from.

Thompson's authorities crowd the drawer far past this one tale — Icelandic and Korean and Japanese and North American Indian caps, Finnish wishing-hats, a Chinese entry — most of them citations that wait unread, with only the Greek witness wired to served text today. Yet the cap of Hades is the clearest of all: the magic garment whose whole virtue is a canceling, a hood that grants not power but absence. What is it that so many peoples imagined the same way — that the deepest advantage is not to strike unseen, exactly, but simply not to be there to be struck back?