Everyone's aware of Psi Cops (all of whom are P12s, though not all P12s are Psi Cops). Example: Al Bester (P12). Most people are also aware of commercial telepaths, who are on the weaker end of the spectrum. Examples: Talia Winters (P5) and Lyta Alexander (also a P5, before the Vorlon incident). But what do these numbers really mean?
P-ratings are a measurement of "telepathic strength," though it's a little more complicated than that. (It always is, right?)
- P-ratings range from 1-12 (with super rare exceptions, for example Kevin Vacit (P13), and Lyta Alexander after the Vorlon incident (P20? P-who-the-heck-knows?)).
- Latent telepaths (for example Susan Ivanova) are not rated - they are legally normals. There is no "P0."
- P-ratings are not linear - the difference between a P11 and P12 in strength is much larger than between a P1 and a P2, and qualitatively, the experiences between ratings differ as well (see below). On occasion, people fall between ratings (having some traits of the higher rating and some of the lower), and are rated in between. Bester's lover Carolyn was rated P11/P12.
- P-ratings rarely change. On occasion, however, young people will develop telepathy in stages, for example developing to P2 level and then later developing to P4 level. Low level telepaths from telepath families (such as Lyta's mom) are therefore monitored by the Corps for a period of time, in case they develop again. Low-level telepaths from normal families may be monitored as well, depending on Corps' resources.
- P-ratings are far from the whole story on telepathic ability. One's training, one's skill, and one's life experience is in many instances at least as important, if not more so, than one's P-rating in determining what one is capable of doing.
- This isn't a simple game of "higher numbers beat lower numbers." If telepathy were like gravity, P-rating would be like one's "mass," and the dent in "telepathic space time" one makes. That has little to do with skill, intelligence, instinct, combat ability, common sense, and so on. Even in cases of actual psi combat it's not that simple, because there's a thousand ways a more skillful person can outwit a less skillful person. (Stephen Walters, aka The Black Fox, was the head of the Underground for a generation, and he was only a P8. He was widely (and inaccurately) believed in the Corps to have been a rogue Psi Cop, but he was actually a P8. Also, after the Telepath War, when most experienced Psi Cops were in prison or dead, and EABI sent fresh-out-of-school P12 kids after Bester, this ended really badly for all of those kids.)
Now, with that aside, what do these ratings really mean?
- Latent telepaths: As Ivanova explains, she is able to feel and (weakly) block a casual scan. She could feel when her mom made contact with her, and in the context of that relationship, could feel her mom's feelings, at least when her mom chose to share them. Outside of the context of that relationship, however (or in theory with another telepath, since we're talking in generalities about latents here), Ivanova, like other latents, doesn't experience others' thoughts or feelings. In summary, latents are normals who can feel telepaths.
- P1s and P2s: These folks are in the zone between "normals" and "telepaths." Legally they are telepaths under Earth Alliance law (as written by normals), but they aren't strong enough be in the Corps (under Corps regulations, written by telepaths). In the early days of the MRA, they were legally treated the same as all other telepaths, but this proved to be an inefficient use of resources as these people weren't telepathic enough to do "telepath jobs" in the new system.
P1s and P2s can sense others' minds to some degree, but not consistently or specifically enough for membership in the Corps. They may absorb the emotions of others around them, but be unable to distinguish what is "theirs" from what is "someone else's." And while they may have general impressions from others, or pick up on strong emotions, they are unlikely to feel specific information in others' surface thoughts. (YMMV - someone may know what his or her partner of twenty years is thinking, but that's not the same as feeling the surface thoughts of a person one literally just met.) P1s and P2s can't communicate telepathically, though they may be able to understand if someone 'casts to them. In summary: It is consistency and specificity that distinguishes the P1s and P2s (not full telepaths) from the P3s and P4s (full telepaths, low end).
- P3s and P4s are surface thought telepaths. In addition to picking up general impressions and strong emotions, they are also aware of specific information (associations, visual, musical, what someone's reading, etc.) in the surface thoughts of others. They have no confusion about what is "theirs" vs. "someone else's." Awareness of others' thoughts often occurs unconsciously; with training, they have some control over what they let in. They can sense both other telepaths and normals, and both people familiar to them and people they've just met (P4s, obviously, with more consistency than P3s). They may at times feel others' physical sensations. They can engage in limited telepathic communication (both ways), can feel if someone is making contact, etc. They may also have some ability to affect the minds/emotions of others around them, in specific limited contexts.
They cannot reach into someone's mind and "extract" information. While (like any telepath) they can strategically ask questions and watch another's thoughts and feelings, and then tailor follow-up questions as a result (or even trick someone into revealing information), they can't actually "scan" anyone (and with P4s, if they can, the ability is rare and inconsistent).
When P3s and P4s do experience higher-level telepathic experiences (e.g. to actively connect mentally on a deeper level (e.g. "scan"), or unconsciously to absorb another's memories), it will usually be limited to specific, intimate contexts (familial, sexual, or otherwise).
Telepaths rated P3 or above make up fewer than 1 in 1,000 in the general population. (Mind War states that 1 in 1,000 humans "has telepathic ability" - I read this to mean that 1 in 1,000 is strong enough to get a rating, not that 1 in 1,000 is strong enough to join the Corps. That number would be smaller, though perhaps not as small as 1 in 10,000, as originally stated.)
- P5s are distinguished from P4s mostly by their ability to actively scan others for information. Talia Winters, for instance, worked as a court telepath and conducted "before and after" scans of prisoners before "death of personality" - this would be impossible for weaker telepaths, even with training. P5s (and above) can actively scan people and retrieve very specific information not just from their surface thoughts, but from their memories as well. They have considerable control over what they "let in" and what they filter out. They may be able telepathically to knock a normal unconscious and cause a migraine, though not with any permanent damage (unless the normal injures him/herself by falling!).
P5s, like weaker telepaths, also require line of sight (and generally proximity) in order to sense things from others. They usually work commercially, in the courts or for the Corps. They don't work in the jobs that require higher level telepathic ability (search and rescue, special ops, as Psi Cops, etc.), though they may work in Metapol as profilers or other assistants to Psi Cops.