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The Operator Symbol. It's a circle with an "X" through it (This one ⦻). It originated in Marble Hornets, where the Slender Man is actually referred to by the creators as "The Operator". It has become the calling sign of the Slender-verse.
➺So what is the symbol?
The closest thing that I've found to information on what that means is that it's the tensor product symbol, used in bilinear operation.
➺The two commonly accepted theories of what it does are complete opposites:
*One of the theories is that it repels Him.
*The other is that it draws Him to you.
*M from the blog "The Tutorial" theorizes that the Slender Man exists on another plane, and if he's not being observed, he can't teleport. The symbol functions as a sort of eye that keeps him locked into reality when he's being observed by it.
*Other theories say that the symbol is an eye... Some believe that they're an interdimensional eye that allows the Slender Man to see through them. So even if people agree that it's an eye, they may disagree on whether it's a good idea or a bad one.
*Another theory on it's meaning is that it's a face. More specifically, the "X" through the circle represents the lack of a face, as we see in one of the pages. If this is true, then it's quite possible that it has no significance at all. The people drawing it are drawing it just to draw it. All it means is that it's his symbol.
*One common theory about Slendy is that he doesn't exist unless you're thinking about him. What happens when you see the Operator symbol, once you've been exposed to the mythos? Exactly. This would fit under the "drawing" theory, only it's the thoughts that draw him and not the symbol itself.
This being is adapting itself to it's environment, but it is a metaphysical environment, not a real one. Belief empowers it, and it chooses what defines it; it chooses a look which stays in the mind, which is powerful.
This explains why it has such a strong connection to the Operator Symbol as well as the terms which are connected with it.. It's altering to a metaphysical setting, resulting in it an ideal target for the myth to emerge from. A plague.
➺Outside of the Slenderman mythos
According to Porphyry of Tyros, the Egyptians used an X within a circle as a symbol of the soul; having a value of nine, it was used as a symbol for Ennead.
Sources: https://slenderbloggins.wordpress.com/2010/11/25/the-operator-symbol/
https://nildesperandumnilfortuna.blogspot.com/2010/09/symbolsfaith-ii.html
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