Psychic paper

Psychic paper was a blank, white card that had special properties. When shown to a person, it could usually induce them to see whatever the user wished them to see printed on it.

Users
The Doctor was given psychic paper while employed as an agent by the Time Lords' covert organisation, the Celestial Intervention Agency, the CIA having developed the technology. (PDA: World Game)

The Doctor, in his Ninth, Tenth and Eleventh incarnations, habitually carried it, as did Captain Jack Harkness when Rose Tyler and the Doctor first met him in World War II London. Jack said that the paper was a new technology put into use by the Time Agency which had employed him. (DW: The Empty Child)

Properties
Usually, it showed what the holder of the paper wanted the person reading the paper to see (DW: The End of the World), or vice-versa (DW: The Idiot's Lantern). It could even be used to open doors in place of a keycard (DW: Army of Ghosts), and the Doctor once used it in lieu of an Oyster card to pay before boarding a bus in London. (DW: Planet of the Dead)

In the hands of an untrained user the paper was prone to displaying facts they subconsciously wanted the reader to be aware of, often leading to embarrassment. (DW: The Empty Child)

Members of the Torchwood Institute received basic psychic training and as a result were rendered immune to the effects of the paper. (DW: Army of Ghosts) Individuals with powerfully creative minds also seemed to be immune to the effect of the paper, although the Doctor appeared to believe this only occurred in truly exceptional cases. (DW: The Shakespeare Code)

Apart from its usual function of subterfuge, the paper could receive messages from beings of sufficient psychic ability, such as the Face of Boe and, to the Doctor's initial surprise, Professor River Song. (DW: New Earth and Silence in the Library) The Atraxi also sent the Doctor a message telling him that Prisoner Zero had escaped. (DW: The Eleventh Hour)

When used by an individual with psychic powers, the psychic paper would sometimes bounce the individual's powers back on them. For example, when used by Waechter, it allowed him to see his own future. (NSA: The Story of Martha: The Weeping)

When wet, the psychic paper acts rather differently and is unable to translate the Doctor's thoughts properly. (NSA: Wetworld)

When a parking inspector signs the Doctor's psychic paper, he believes it won't work anymore until Amy Pond turns it around the other way, causing messages to appear back to front. (NSA: Apollo 23)

When the Doctor attempted to pass himself off as a "universally-renowned responsible adult" to the young Kazran Sardick, the psychic paper only displayed squiggly lines, the Doctor realizing that he had finally said something so unbelieveable that the paper "shorted out". (DW: A Christmas Carol).

Resistance
Not everyone falls for the psychic paper. People, or groups, known to be immune to its illusions include:


 * Torchwood One personnel
 * Due to psychic training. (DW: Army of Ghosts)


 * William Shakespeare
 * Apparently due to Shakespeare's incredible mind. (DW: The Shakespeare Code)


 * Ernest Shackleton
 * Shackleton states that such "mentalism" tricks don't work on him. The Doctor surmises it's due to the explorer's driven and determined nature. (DWM: The First)


 * Eve
 * Possibly from being an android, possibly from her determined nature. (NSA: The Last Dodo)


 * Rosanna Calvierri
 * Likely due to her knowledge of the Time Lords, or the fact that she wasn't even humanoid in nature. (DW: The Vampires of Venice)

Suggestibility Paper
The Doctor also carries rolled up in his sock a sheet of "Suggestibility paper". Presumably it is of a similar concept to psychic paper, but its abilities as yet are unknown. (IDW: Fugitive)