# Telepathy
Telepathy, sometimes referred to as ESP, is a passive property in NetHack that allows you to detect non-mindless monsters, even if you cannot physically see them.

Contents

1 Description

1.1 Intrinsic sources
1.2 Extrinsic sources

2 Strategy

2.1 Telepathy and monsters
2.2 Artifacts
2.3 Zen conduct

3 Origin
4 Messages
5 Variants

5.1 SLASH'EM
5.2 GruntHack
5.3 SporkHack
5.4 UnNetHack
5.5 dNetHack

5.5.1 Telepathy and items
5.5.2 Artifacts

5.6 DynaHack
5.7 FIQHack
5.8 xNetHack
5.9 notdNetHack
5.10 EvilHack
5.11 SlashTHEM

6 References

Description
 The range of unblind extrinsic telepathy is indicated by flags. In comparison, the range of warning is slightly greater, and is indicated by question marks.
Telepathy can be acquired as both an intrinsic and extrinsic property through various means, and function somewhat differently depending on their source. It is possible to have both forms of telepathy independent of each other, with the extrinsic property taking priority over the intrinsic.

Intrinsic telepathy allows you to detect all non-mindless monsters on the current level, but only while you are blind.[1]
Extrinsic telepathy allows you to detect all non-mindless monsters within a radius of 8 squares, and additionally displays all non-mindless monsters on the current level while you are blind.[2]
Only three monsters have intrinsic telepathy:

e&nbsp;floating eye
h&nbsp;mind flayer
h&nbsp;master mind flayer
If a mind flayer or master mind flayer is near enough to you that it can see you, they will attack you with a long-distance psionic blast for d15 damage; possessing telepathy allows them to detect you and land psionic blasts more often. Peaceful and tame flayers will instead produce more "soothing" waves that have no effect on you (though tame mind flayers will also target hostile monsters with damaging blasts). Polymorphing into a mind flayer allows you to use this ability with the #monster extended command.

Intrinsic sources
Intrinsic telepathy can be acquired through the following methods:

Eating the corpse or tin of a floating eye (100% chance) or a (master) mind flayer (50%)
Eating an amulet of ESP
Polymorphing into a monster with telepathy (i.e., a floating eye or either mind flayer)
Praying to your god under certain circumstances
Intrinsic telepathy can be lost if you are either hit by the gremlin's intrinsic-stealing attack at night, or else commit murder as a lawful or neutral character.

Extrinsic sources
Extrinsic telepathy can be acquired through the following methods:

Wearing an amulet of ESP, including the Eye of the Aethiopica, or a helm of telepathy
Carrying any of the following artifacts:
The Longbow of Diana
The Orb of Detection
The Magic Mirror of Merlin
The Platinum Yendorian Express Card
Strategy
Both forms of telepathy are considered among the more useful methods of detecting monsters, as they allow for much safer movement (especially involving pets) if blinded unexpectedly, e.g. an exploding yellow light. Human characters in particular will appreciate telepathy as a way of compensating for a lack of infravision. However, the late game contains dangerous mindless monsters such as the stronger golems and green slimes; telepathy is best supplemented with other forms of monster detection, such as warning.
Telepathy also grants players the ability to prepare for or path around especially dangerous monsters depending on their strategy. The intrinsic is usually considered sufficient for such purposes - it is sought out enough that some Monk players will break vegetarian conduct and take the -1 alignment penalty in order to eat a floating eye. Telepathy can also be obtained from prayer, though this requires management of high Luck and a co-aligned altar, and is generally the ideal source of the intrinsic for foodless or vegan players; extrinsic telepathy is preferred for conducts that rely heavily on awareness of your surroundings, such as pacifist. 

Telepathy and monsters
Players very often keep a blindfold or towel on hand for telepathic characters so they can blind and unblind at will to scan their current level; this is frequently done to seek out specific monsters, e.g. looking for lost pets, discerning the exact location of Medusa on some variations of her level, or even finding the Riders on the Astral Plane. As both forms of telepathy display monsters while blind, some players may mistake this as being exclusive to the intrinsic property.
The mental blasts of mind flayers only occur if you currently have telepathy, and can be counteracted by temporarily removing or stashing whatever extrinsic telepathy source you have. This is not always applicable, and is often unnecessary in practice anyway - the damage is usually negligible outside of scenarios involving early minesflayers, or else escaping from other monsters while at low HP, and it is very rare to deal with multiple mind flayers at once outside of creating them via reverse genocide. See the article on mind flayers for other strategies dedicated to handling them in combat.
When employing boulder forts against specific threats, telepathy is useful to scan the area and keep track of their location. Seeing a monster via extrinsic telepathy also makes it a valid target for the cone of cold and fireball spells when cast at Skilled or higher level in attack spells.

Artifacts
Artifact wishes that come after most (or all) other essential properties are covered will usually prioritize sources of telepathy. With the exception of the Eye of the Aethiopica, all artifacts that give telepathy grant the property while carried, preserving slots for other properties. Monks in particular aiming to preserve vegetarian or vegan conduct may opt for these artifact wishes.

The Magic Mirror of Merlin is lawful and also gives magic resistance, and is an ideal wish target for lawful Monks.
The Platinum Yendorian Express Card is neutral also provides magic resistance and half spell damage, and can be invoked for charging, making it one of the most desirable artifact wishes.
The Eye of the Aethiopica is neutral and a highly-desired artifact for most spellcasting-focused characters - carrying it alone grants half spell damage and faster energy regeneration, while wearing it also provides telepathy and magic resistance at the "cost" of the amulet slot.
Zen conduct
Main article: Zen
Telepathy is an absolutely vital intrinsic for zen conduct play: starting the game being unable to see monsters while blind will leave you at the mercy of molds and anything with a passive attack, or more likely the wrath of a peaceful monster that you angered by bumping into them. In particular, telepathy enables zen characters to safely navigate shops without angering the shopkeeper, which is almost always game-ending if it occurs.

Origin

Wikipedia has an article about:
Telepathy

Wikipedia has an article about:
Extrasensory Perception

Telepathy is the ability to transmit information from one person's mind to another's without using any known human sensory channels or physical interaction. The term was first coined in 1882 by Frederic W. H. Myers, a founder of the Society for Psychical Research (SPR), and is among the more popular concepts of thought-reading; the concept emerged in the context of significant advances in physical sciences of the era, where scientific concepts were applied to mental phenomena in the hopes of understanding other events considered paranormal (i.e. beyond the understanding of science at that time). As the scientific disciplines progressed in development, telepath experiments have been criticized for a lack of proper controls and repeatability, and the topic is generally considered pseudoscience due to lack of good evidence.
ESP is an abbreviation for ExtraSensory Perception, which refers to any perception not gained through the body's physical senses. The term was coined by parapsychologist Joseph Banks Rhine in his 1934 book (also titled Extrasensory Perception), and encompasses a number of purported psychic abilities, including telepathy and clairvoyance, among others. 

Messages
You feel a strange mental acuity.
You gained intrinsic telepathy from eating a corpse.
You feel in touch with the cosmos.
As above while hallucinating.
A wave of psychic energy pours over you!It locks onto your telepathy!
A (master) mind flayer is on the same level as you and attacking you via telepathy; if you are telepathic, the second line occurs and you take d15 damage.
It feels quite soothing.
As above, but the mind flayer is peaceful or tame.
You are telepathic.
You have telepathy and are viewing your attributes via enlightenment.
Variants
In some variants with object properties systems, telepathy is an applicable property for some items.
Of note is that Itlachiayaque from the patch by L replaces the Orb of Detection as the Archeologist quest artifact in some variants; Itlachiayaque also provides telepathy in most of these instances, with the exception of its appearance xNetHack.

SLASH'EM
In SLASH'EM, the potion of ESP can be quaffed to grant intrinsic telepathy temporarily if uncursed; a blessed potion grants the intrinsic permanently, while a cursed potion removes the intrinsic if you have it. This provides a viable source of the intrinsic for vegan players, especially Monks.
The Crown of Saint Edward is an artifact helm of telepathy that serves as the Yeoman quest artifact, and additionally provides magic resistance and half spell damage, making it another enticing artifact wish for lawful players.
In the Jedi role patch for SLASH'EM, player Jedi gain telepathy at experience level 3.

GruntHack
In GruntHack, various objects can have the telepathy object property, which grants extrinsic telepathy while it is wielded or worn; this property cannot occur on projectile weapons.

SporkHack
In SporkHack, the tinfoil hat is a Randomized appearance for helms that causes the helm to block telepathy in both directions while worn: this both suppresses telepathy from any other sources (including the helm itself, if it is a helm of telepathy) and protects you from the psionic blast of a mind flayer.

UnNetHack
In UnNetHack, the tinfoil hat is a non-randomized helm that functions the same as it does in SporkHack.

dNetHack
In dNetHack, elven and incantifier Anachrononauts start the game with telepathy, as do Madpeople of all races. There are also more monsters with mind blast attacks such as star spawn and elder brains - Great Cthulhu also has a unique mind blast attack that can inflict a "dream" madness on you. These may make telepathy more undesirable for certain playstyles, and makes extrinsic sources somewhat safer in comparison since they can be removed, whereas the most reliable way to lose intrinsic telepathy is to commit murder, which has far stricter consequences in dNetHack.
Starting telepathy is a great boon for elven and incantifier Anachrononauts in particular early on, but makes their quest branch especially difficult: several mind flayers will be among the hostile monsters on the home level, and the barrage of mind blasts can easily whittle down an incantifier's HP. Worse yet, genocide does not apply to the quest branch in dNetHack, preventing you from preemptively removing them.

Telepathy and items
Weapons in dNetHack can have the "psionic" or "rustling" object properties, giving them extra damage against telepathic creatures:

Psionic weapons deal bonus damage to telepathic creatures, equal to double their base dice and three times enchantment; this bonus has a 1⁄5 chance of activating versus non-mindless monster without telepathy, and will never occur when attacking a mindless monster. A weapon with a psionic property will be visible to you if you are telepathic (even when not blinded), and will be described as "whispering" - the psionic property will be visible on the weapon when it is fully identified.
Rustling weapons are weaker and deal +2d12 bonus damage to the same vulnerable targets as psionic weapons. The rustling property can only be seen if you are telepathic, even if the weapon is fully identified.
Telepathy can also interact with certain other objects such as crystal skulls and lightsabers.

Artifacts
In addition to the artifacts retained from vanilla NetHack, the following dNetHack artifacts provide telepathy in some form:

The Palantir of Westernesse, when carried
The Rogue Gear-Spirits, when carried
Itlachiayaque, when carried
The All-seeing Eye of the Fly (a large helm of telepathy), when worn
DynaHack
In DynaHack, telepathy is one of the many object properties available.

FIQHack
In FIQHack, telepathy is available as an object property similar to GruntHack, but generates 1⁄10 as often as most other properties. Dipping a compatible object in a potion of wonder has a chance to imbue it with the telepathy property.

xNetHack
In xNetHack, telepathy is cooperative: if you and another monster have intrinsic or extrinsic telepathy, you will be visible to each other. Gnomish wizards, kobold shamans and orc shamans are telepathic, and may infrequently grant intrinsic telepathy when eating their corpse or tin.

notdNetHack
In notdNetHack, the details listed for dNetHack are still largely applicable. Additionally, the Illithanachronounbinder role starts with extrinsic telepathy, since they are beings that were made to host illithid larvae that parasitically converted them into illithids.

EvilHack
In EvilHack, telepathy is cooperative as in xNetHack: you and others monsters can see each other if you both have telepathy. Some spellcasting monsters such as goblin shamans and gnomish wizards are given telepathy, and eating their corpse or tin has a chance of granting the intrinsic.
The illithid (or mind flayer) is a playable race that always starts with intrinsic telepathy; Monks can also gain the intrinsic by reaching XL 20, further removing the need to rely on eating corpses and incur alignment penalties (which are harsher in EvilHack). Various objects can also have the ESP object property which grants extrinsic telepathy when they are worn or wielded, similar to GruntHack.
The Crossbow of Carl is the gnomish counterpart to the Longbow of Diana, and thus also grants telepathy.

SlashTHEM
In SlashTHEM, the Jedi patch is integrated into the base game, and Jedi gain telepathy at XL 3. Illithids are also a playable race, and start with intrinsic telepathy.
There are also additional artifact sources of telepathy alongside the ones retained from SLASH'EM:

The Deluxe Yendorian Knife, the Chef quest artifact
The Gourd of Infinity, the Drunk quest artifact
The Nethack Sources, the Geek quest artifact
The Lockpick of Arsene Lupin, the Locksmith quest artifact
The Whistle of the Warden, the Officer quest artifact
The Golden Whistle of Nora, the Zookeeper quest artifact
References

↑ include/display.h in NetHack 3.6.7, line 30

↑ include/hack.h in NetHack 3.6.7, line 18