Mark 9:17-19 (New King James Version)
17 Then one of the crowd answered and said, “Teacher, I brought You my son, who has a mute spirit. 18 And wherever it seizes him, it throws him down; he foams at the mouth, gnashes his teeth, and becomes rigid. So I spoke to Your disciples, that they should cast it out, but they could not.”
19 He answered him and said, “O faithless generation, how long shall I be with you? How long shall I bear with you? Bring him to Me.”
The word doppelgänger is a loanword from German: Doppel (double) and Gänger (walker). It was first used by Jean Paul in the novel Siebenkäs (1796), and explained by a footnote.
As is true for all other common nouns in German, the word is written with an initial capital letter. In English, the word is conventionally uncapitalized (doppelgänger). It is also common to drop the diacritic umlaut, writing "doppelganger."
 Scientific and philosophical investigations
 Left temporoparietal junction
In September 2006, it was reported in Nature that Shahar Arzy and colleagues of the University Hospital, Geneva, Switzerland, unexpectedly had reproduced an effect strongly reminiscent of the doppelgänger phenomenon via the electromagnetic stimulation of a patient's brain. They applied focal electrical stimulation to a patient's left temporoparietal junction while she lay flat on a bed. The patient immediately felt the presence of another person in her "extrapersonal space." Other than epilepsy, for which the patient was being treated, she was psychologically fit.
The other person was described as young, of indeterminate sex, silent, motionless, and with a body posture identical to her own. The other person was located exactly behind her, almost touching and therefore within the bed on which the patient was lying.
A second electrical stimulation was applied with slightly more intensity, while the patient was sitting up with her arms folded. This time the patient felt the presence of a "man" who had his arms wrapped around her. She described the sensation as highly unpleasant and electrical stimulation was stopped.
Finally, when the patient was seated, electrical stimulation was applied while the patient was asked to perform a language test with a set of flash cards. On this occasion the patient reported the presence of a sitting person, displaced behind her and to the right. She said the presence was attempting to interfere with the test: "He wants to take the card; he doesn’t want me to read." Again, the effect was disturbing and electrical stimulation was ceased.
Similar effects were found for different positions and postures when electrical stimulation exceeded 10 mA, at the left temporoparietal junction.
Arzy and his colleagues suggest that the left temporoparietal junction of the brain evokes the sensation of self image—body location, position, posture etc. When the left temporoparietal junction is disturbed, the sensation of self-attribution is broken and may be replaced by the sensation of a foreign presence or copy of oneself displaced nearby. This copy mirrors the real person's body posture, location and position. Arzy and his colleagues suggest that the phenomenon they created is seen in certain mental illnesses, such as schizophrenia, particularly when accompanied by paranoia, delusions of persecution and of alien control. Nevertheless, the effects reported are highly reminiscent of the doppelgänger phenomenon. Accordingly, some reports of doppelgängers may well be due to failure of the left temporoparietal junction.
See monothematic delusion for a detailed description of various psychological problems including the syndrome of subjective doubles, which may be related to the doppelgänger. See also out-of-body experience and the Third Man phenomenon.