A civilization stands or falls on whether words in the mouth of a witness can be trusted. The command to inquire diligently is a command to take the truth with deadly seriousness. A false accusation turns the machinery of justice into an instrument of murder. Careless judgment rots the ground.
The liar in the witness box attacks his brother and the whole order at once. When testimony can be faked without consequence, no one is secure, and the strong simply invent crimes for the weak. So the inquiry must be patient and merciless in its search. Guard the honesty of speech, or watch the world it holds together come apart.