Everybody Lies is House's credo, and usually governs House's diagnostic technique and personal life. It is a phrase constantly used in the show in many contexts, particularly when a very unlikely diagnosis would only make sense if someone were not telling the truth.
House's credo often gives him good guidance, as in many episodes the underlying diagnosis is often lacking either because someone has lied outright without provocation, has lied when asked about a possible source of the disease directly (particularly sexual infidelity) or did not disclose a particularly important fact that they thought was unimportant or too embarrassing.
However, even House agrees that his credo has its limits. He notes that people will only lie for a good reason, and that many people will lie only in some extreme situation that they feel is incredibly important. He also has identified two individuals who almost never lie, his father and Dr. Cameron. This is not to say that House sees honesty as a weakness. Where House uses lies and manipulation to get his way with patients, Cameron uses empathy and forthrightness and can sometimes get the same results. House also trusts his ex-girlfriend Stacy implicitly, in one case preferring to believe her rather than her sick husband. When Cameron challenged him by asking "whatever happened to everybody lies?", House answered "I was lying when I said that".
House also has another important credo - the symptoms never lie. However, even he agreed that they sometimes do.
House has even identified a disorder, Korsakoff's syndrome, which embodies his credo. Korsakoff patients have no memory of recent events and will put together explanations from visual clues to answer questions. House tells students to treat every patient as if they have Korsakoff's.
Surprisingly, "Everybody lies" is also the credo of one of House's arch nemesis, Detective Michael Tritter, who treats everyone he questions as if they are lying.
'Everybody Lies' is also the semi-name for the Pilot.
See The List of Lies