Legal basis: Article 60 of the Criminal Procedure Law of People's Republic of China (PRC). Anyone who knows the case has the obligation to testify. The witness must be someone who knows the case, so it is specific. It is impossible for a person who doesn't know the case to be a witness. Witnesses are generally temporarily selected and invited by investigators at the scene of litigation activities, and they are not required to know the case. In other words, investigators have great randomness and selectivity in choosing and inviting citizens as witnesses, and almost any bystander on the scene may be invited as a witness. After being invited to be a witness, a citizen becomes a specific person who knows about relevant litigation activities, similar to a witness. But what the witness knows is only the litigation activities of the investigators, not the case itself.