Truth
Truth was a belief or statement that was true or in accordance with fact or reality. To lie was to conceal the truth.

A means of determining the truth was referred to in the great detective's credo, "we must fall back on the old axiom that when other contingencies fail, whatever remains, however improbable, must be the truth." (; ; )

According to Spock, evil sought to maintain power by suppressing the truth. ()

Many myths, such as that of Eden, were said to be based on truth. ()

The truth is 
Guinan once told Ro Laren that she believed that "truth is in the eye of the beholder." When Ro questioned the accuracy of her statement, asking if that was "supposed to be beauty?" Guinan responded "truth, beauty. It works for a lot of things." () Elim Garak shared the same belief, sharing with Julian Bashir that "truth, Doctor, is in the eye of the beholder. I never tell the truth because I don't believe there is such a thing." () Garak held the same belief about treason, which he also maintained "like beauty, is in the eye of the beholder." ()

Another time Garak argued that "the truth is usually just an excuse for a lack of imagination." This, however, was stated some time after he complained, "why is it no one ever believes me, even when I'm telling the truth?" To which, Dr. Bashir recalled the story of "The Boy Who Cried Wolf", summarizing that "if you lie all the time, nobody's going to believe you even when you're telling the truth." However, Garak disputed the traditional interpretation of the fable, suggesting an alternative moral: "Never tell the same lie twice." ()

Revealing the truth 
During the 23rd century, the Federation possessed technology that was capable of determining whether or not an individual was telling the truth. When using this device, known as a verifier, a testifier would sit and place their hand on a plate; a version also existed without use of the plate. As they spoke, the verifier would immediately detect any deviation from factual truth was relayed to the computer and reported. () The verifier could also be used in conjunction with truth serum. ()

During this time, the Klingons possessed a more barbaric device called a truth finder that was capable of forcefully extracting the truth from its users. The Klingons found it to be a more suitable alternative to "such crude methods", such as truth drugs. ()

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Philosophy