context: The film opens with two primary characters: university academic Margaret Matheson (Sigourney Weaver), who investigates claims of paranormal phenomena, and her assistant Tom Buckley (Cillian Murphy), a physicist. The audience is provided with an insight into the world of the opening section's primary characters while concurrently observing the public re-emergence of a psychic, Simon Silver (Robert De Niro).
  The ending of the film's first half is signified by the sudden death of Matheson from a chronic vascular condition at the same time that one of Silver's comeback performances takes place; an incident that is particularly significant due to the death of a former nemesis of Silver, a skeptic who investigated the psychic's work, under similar circumstances. Matheson also had a previous encounter with Silver. She recounted a public meeting when Silver had, for an instant, got the best of her by bringing up the subject of her son's spirit (her son was in a vegetative coma and on life support). Matheson agrees only to appear on a televised panel in anticipation of Silver's return. Prior to her death, Matheson refuses to co-operate with Buckley's insistent call to undertake another investigation of Silver, warning Buckley against such an undertaking due to her previous experience with the psychic.
  However, following Matheson's death, the assistant becomes increasingly obsessed with investigating Silver for the purpose of exposing the popular psychic as a fraud. During Buckley's efforts to reveal Silver's large-scale trickery, a series of inexplicable events occurs â electronic devices explode, dead birds appear, and Buckley's laboratory is vandalised. Buckley's paranoia intensifies, as he believes Silver is behind these incidents. Buckley's calm and rational disposition eventually degenerates into an obsessiveness that resembles the late Matheson's intense antipathy to paranormal claims. As part of the introduction to the climactic section of the film, Silver agrees to participate in an investigation proposed by an academic from the same university that Matheson was employed by, and Buckley joins the observation team for the tests.
  In the final moments of the film, Buckley's assistants manage to reveal the manner in which Silver defrauds the public through a close analysis of the test footage accumulated by Buckley from the university's investigation. At the same time, Buckley exposes Silver at one of the psychic's public performances, and Silver is left dumbfounded. Buckley then reveals to the viewer that he actually possesses paranormal abilities and has been responsible for the inexplicable incidents that have occurred during his investigation of Silver. In a letter to his late mentor, Buckley explains a realisation in which he arrives at an understanding that his decision to work with Matheson, despite the possibility of loftier career opportunities as a physicist, was the result of an unconscious attempt to seek others like himself; the revelation clarifies that Buckley's choices were made in spite of his conscious denial of the existence of paranormal activity (such denial is touched on earlier in the film, whereby the character implies that he chose this career because his mother was delayed from seeking critical medical treatment due to advice from a fraud psychic). The letter to Matheson ends with regret that Buckley denied her the consolation of knowing that there is something more, and that now she deserved even more, "everything." Buckley then turns off the life-support machine that is keeping Matheson's son alive. He then walks out of the hospital and concludes his letter to the deceased Matheson, "You can't deny yourself forever".
question: Who are professional skeptics?
answer: Margaret Matheson and Tom Buckley