We covered a lot of ground in World Building With Friends: Telepathy. Oh boy did we cover a lot of ground. So instead of making people pick through the conversations, how about I quick sum up what I've concluded and is going into the world building. At least at this point in the project.
Telepathy is a natural process that many animals do in the wild. Something about consciousness allows two or more individuals to resonate and form a single mind. While we see it most often in mammals and birds (at least between offspring and parent), it's also seen in social insects.
In the case of insects, it's more a case of emergent consciousness where simpler parts add up to more than the whole. For vertebrate nervous systems, the opposite happens. A group of individuals have to suppress their individuality to operate as a single unit. Which is why insects generally are seen operating in large groups, there is virtually no capacity for individuality in any of the constituent units.
Several theories exist about the exact mechanisms of telepathy. What is known is that the signals are carried outside of reality on the spiritual plane, where time and space do not exist as concepts. Despite low-level telepaths needing to be in physical contact, this is merely to establish resonance. High level mages can trigger telepathy between another willing party of the same level across interplanetary distances, and experiments have shown that this communication can even span across time.
Spontaneous Telepathy is often unconscious, and generally only contains strong emotion. In human beings, spontaneous Telepathy is not uncommon between mothers and developing fetuses. As the child's mind develops and starts formulating individual thoughts, the connection breaks down. Mothers are often unaware of the connection, because their feelings are the dominant force in the relationship. Spontaneous Telepathy between twins is also a common occurrence.
The best research shows that frequent physical contact allows individuals to resonate naturally, albeit uncontrollably.
The first stage in the development of telepathy is to tune oneself to the emotions of others. This requires a suppression of one's own conscious thoughts, and starting to open oneself to the spirit plane. Level 1 telepaths can read the actual emotional state of a willing individual in the vicinity, and the strong emotions of an unwilling individual.
It's often claimed that level 1 telepaths can discern truth from lie. But this is not accurate. They can discern that an individual is masking emotion, and that they are unwilling (or unable) to establish a telepathic connection. But this is not indicative of a lie, per se.
Several medical conditions impair natural telepathic abilities. And while many cases of blocking a telepathic link are attempts at subterfuge, there is also this nice thing called privacy.
Low level telepaths are notoriously noisy with their signal. A high level telepathic can exploit a connection to an unskilled telepath to read far more information than the low level ever intended to share.
A level 1 telepath connected to another level 1 telepath can exchange feelings, in the same way people can converse with simple facial expressions. In computer text messages we approximate this with the Emoji.
Level 2 telepaths can share fully formed ideas between willing individuals of the same skill level. The content of the idea doesn't matter. It can be a picture, a word, an abstract concept. They do have to take turns in the conversation, and both minds must focus to maintain contact.
Level 3 telepaths can foster connections between themselves and beings of a lower telepathic level that they are within visual contact with. They can also establish connections with acquaintances at any distance.
Mages with familiars are operating at level 3.
Note: Certain medical conditions prevent telepathy. No amount of telepathy skill allows these individuals to participate in telepathic links.
Level 4 telepaths can foster connections between two other individuals, regardless of their level. In professional circles, they operate like switchboards. They have the ability to induce resonance on strangers within physical contact. They can also hand off a psionic link to another level 4 or above telepath.
Level 4 telepaths can contact other level 4 telepaths, even if they are complete strangers.
Level 4 telepaths can detect thoughts broadcast on the psionic plane.
Level 5 telepaths can induce resonance on an unwilling individual on contact. They can also detect other telepaths operating in their immediate physical vicinity. Within visual contact, they can tap into psionic links surreptitiously.
Level 5 telepaths can broadcast thoughts into the minds of people (willing or unwilling) they are within visual contact with.
For telepathy to work, both minds must be in cooperation. Attempts to impose one's own will generally disrupt the effect of telepathy. While there are records of highly skilled mages combining Telepathy and mind control, the mechanism for entralment was established first. The telepathic bond merely established to pass commands.
It is very common for high level mages to take on an animal familiar. These most common are monkeys, cats, dogs, crows, and parrots. In tropical ocean environments, it is also not unheard of for mages to employ cetaceans, in particular dolphins. Legends tell of reptile and insect familiars, but these are considered very, very unlikely candidates by modern standards. (They can still be trained, but their ability to connect via telepathy is suspect.)
Some artificers have reported success in employing telepathy to control robots and living computers. But this is still considered a unique gift found only in high level alchemy mages. And, as of yet, these claims have not been substantiated to the satisfaction of the Union of Tegical Ethics. They likely exist, but the skill is still too new to be adapted into general purpose teaching material.
All beings of level 4 and above are required to register their ability with the local government, and submit to law enforcement inquiries. While forensic data is hard to collect, when a case of telepathy being misused can be proven beyond a reasonable doubt, it is heavily punished.
Licensed telepaths carry a specially marked government ID.
Surreptitiously breaking into psionic communications is prosecuted like phone tapping. Intimidating another individual over telepathy is considered assault in may venues.
Professional telepaths utilize a noise cancelling device called a "resonator". These devices suppress local noise, and also mask the individual's psionic pattern. All legitimate versions of these devices require individual authentication, and record a log of use. This log may be inspected at any time by law enforcement.