The Silph Scope was initially stolen by Team Rocket, and is obtained from the Rocket Hideout after the player defeats Giovanni. It enables the player to identify the Ghosts that are appearing within Pokémon Tower in Lavender Town, and is required to pass the stairway who is guarded by a ghost Marowak.
In Generation I the item could be stored in a PC and if done so the Ghosts couldn't be identified because the player must have the item in their bag. There's also a glitch in Generation I where you could use a PokéDoll to flee the battle and the ghost will disappear as it was defeated, meaning that you could entirely skip Rocket Hideout. This glitch was fixed for the Generation 3 Remakes.
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