A condemnation that doubled as a description
In the 12th century, John of Salisbury wrote about specularii—people who used reflective surfaces in attempts to discover hidden things, including who had stolen property. A polished surface could be treated as a tool for divination: the user gazed into it in hopes that an image or vision would disclose information ordinary searching could not provide.
John meant to condemn the practice. But describing it in detail also made it easier for readers to picture what the practitioners did and what they hoped to learn. A warning can function as an inadvertent advertisement when it gives a forbidden activity a name, a procedure, and a compelling purpose.
How criticism could spread the practice
The appeal was practical as well as mysterious. Theft left victims with a question—who took the goods?—and divination promised a way to answer it without evidence. That promise could make reflective-surface rituals memorable, especially when a learned writer devoted attention to them. His account may therefore have helped circulate an idea he opposed, though it cannot by itself prove that his writing caused the practice to become popular.
The episode also needs a small distinction: John described gazing into polished surfaces, not necessarily the later, familiar image of someone staring into a crystal ball. The broader method is often called scrying—seeking visions in a reflective or otherwise visually simple surface. His complaint is striking because it shows how an attempt to suppress a custom can preserve its instructions and make it more imaginable to the people who read about it.
