The slan telepathic ability is made possible by the tendrils in their hair. These "pick up" thought waves and relay them down into the brain. Slans without tendrils are therefore unable to read minds. Slan telepathy is purely passive except with other slans.
However, Jommy could use his hypnosis techniques to dominate others— for more details, see Gadgetry: Hypnotism Crystals. Also, thoughts could be recorded and broadcast using Porgrave machines, even between non-telepaths. For more details see Gadgetry: Porgrave Thought Broadcasters.
- Some topics are easier to convey through telepathy than others; technical information seems to be difficult to transmit (137).
- Slans can rapidly exchange large amounts of information directly between their minds (190).
- Some environments can "diffuse" thought waves, hindering a slan's ability to detect them except across very short distances (61).
- People who are asleep "radiate" less thought than those who are awake, but they can still be sensed (18). Their dreams can be read by slans (19).
- Slans can use their telepathic abilities to easily avoid detection, by taking advantage of gaps in people's attention (44, 132).
- Being in contact with a mind is likened to flipping through a book with an infinite number of pages. Recent memories are the easiest to read (111, 135).
- Slans can detect thoughts up to a fourth of a mile away (141). They can also sense the minds of animals and even identify them that way (42). All living things give off a "life wave" that can be sensed by mature slans, and often at great distances (147).
- Even when sleeping, a slan's mind can subconsciously detect danger, and he will awaken to deal with it (39, 141).
- Slans can sometimes even feel the physical sensations others are experiencing (9, 21, 148), as well as seeing through others' eyes and hearing through others' ears (88, 142-3, 148, 169).
- Adults slans mature to the point where their tendrils can oversee their surroundings for many miles in incredible detail, their superior brains being able to handle the massive amount of information coming in (147).
The presence of other minds is sometimes described as a "weight"; this seems to be partly due to number, though imminent danger can be a contributing factor (88). In crowded places, the thoughts of so many minds creates a kind of vague background murmur (5-6, 45) that makes it difficult to distinguish specific minds, though it can be done with effort. This could be compared to trying to filter out a single conversation in a crowded room. Adult slans can easily pick specific thoughts out of crowds and over distances, though children have more difficulty (6, 31).
Emotions come through clearer than thoughts, and often cannot be withheld from slans through willpower. However, emotions also tend to obscure the clarity of other thoughts in the mind, so the emotion is often the only thing that can be read (28, 59), although if the emotions are intense enough other thoughts might leak out for a short time (48- 9). Sudden unexpected events can cause brief emotions that are impossible to "cover up" with a mind shield (32).
Sensing the emotions of others can be overwhelming (18-9, 20, 24, 45); this is intensified by the nature of the emotion (20), the proximity (19), and the number of people feeling it (25, 45). Sometimes this "overload" can manifest itself in the form of physical pain (20, 25).
Fatigue or pain, as well as the emotional state of those around them, can hinder the precision of slans' telepathic ability, regardless of their age (10, 25). Slans, especially children, can become fatigued by intensely focusing their mind on another's (6, 19).
When minds become shrewd and calculating it is more difficult for slans to read them (27).
Speaking focuses the thoughts, and spoken words can often cut off easy access to the other areas of the speaker's mind (111-2).
Much as one might halfheartedly listen to someone else's conversation, so can a slan maintain "casual" contact with someone's mind (66).
Slans can transmit emotions as well as thoughts to one another, making the other slan feel these emotions themselves as if they were their own (7). Slans can instantly sense others of their own kind nearby, and can pick out which individuals in a crowd are fellow slans (39, 46). They can even sense tendrilless slans in this fashion (46-7, 114).
When two slans share a close relationship, their thoughts can freely bounce back and forth and mingle with other, creating the impression that they share a single mind (141-2).
Slans can "open" and "close" their minds to all outside thoughts (16, 93). However, even when a slan chooses to do this, a certain amount of "background noise" always makes its way through (18, 45).
When a person is knocked unconscious, a slan watching that person's mind will notice it "slowing down" until it stops (95). For a brief period after a person looses consciousness, thoughts rise to the surface allowing the slan to read them, obviating any mind shield (108). The sensation of being in contact with a dying mind is different than anything else a slan can experience, leaving no doubt as to exactly what has happened (142).
It is possible to channel individual thoughts to a specific slan (27, 106). The clarity of this "transmission" depends on the mental discipline of the person attempting it. A person with a cluttered mind also lets miscellaneous thoughts into the message (106), while those using mind shields can do so without letting any of their other thoughts out. One does not have to be a slan to direct their thoughts in this manner (27).
Slans have the ability to shield either all thoughts (46) or only specific ones (5, 140) from each other. Doing so, however, is a skill that improves with practice, and those who have never done it before have great difficulty at first (135).
Some individuals, such as tendrilless slans and humans, have disciplined their way of thinking to such an extent that they can "camouflage" their thoughts, making them hard to read or impenetrable even to the most skilled slan (8, 19-20, 22). They do this by several methods, such as thinking about several things at once to make their individual mind blend in with the variety of thoughts from multiple individuals (19-20), or by thinking only specific thoughts on the surface of their minds (22). Sometimes these surface thoughts can come through so clearly that the slan can experience some disorientation when such an individual speaks — it can seem as if the words being spoken were only being thought (22).
Some thoughts always leak from any mind shield; the combined "noise" from such leaks from numerous individuals can be overwhelming in the same way that a smaller group of unblocked minds would be (93). Exceptionally cunning people can use the very faulted nature of mind shields to their own advantage, deliberately leaking out false information (98). Mind shields can be lowered at will, either partially (only allowing a slan access to certain information) or completely (102-3, 111, 136, 171, 190).
Using a mind shield is a basic practice among both true and tendrilless slans (46, 96, 182), but was far less common among humans — specific humans who could do this include John Petty (8), some of his secret police (20), Kier Gray (22), and Jem Lorry (35). There is a difference between the mind shields used by humans and those used by slans; an experienced and skilled slan can spot the difference immediately (144-5, 182).