The truth

The phrase "the truth, the whole truth, and nothing but the truth" is the oath witnesses take in court. It is meant to cover all forms of deceit. 

Here is a breakdown of what each part means: 

• "The truth": A witness must not knowingly tell any falsehoods. This can be complicated by biases or self-deception.

• "The whole truth": The witness must not omit any relevant details. Leaving something out could alter the meaning of the evidence, even if no direct lie was told. This prevents "lying by omission".

• "And nothing but the truth": A witness cannot add false information to an otherwise true statement. This stops them from inventing details that did not happen. 

Significance in the legal system

In countries that follow the English common law tradition, the oath is a part of judicial proceedings. It reminds the witness of the gravity of their words and the serious consequences of lying under oath, which is a crime called perjury.