the thoughts of a witness. This concept can diminish the credibility of evidence.?

Answer 1

Bias can diminish credibility -- so can general competence, mental stability, and many other factors.

Answer 2

It does because each witness can see the same incident but view it in a different way. It is not that the witness is not being truthful, it is that each person looks at things differently.

In my criminal law class the prof had staged a robbery. Two guys in ski masks entered behind the prof. Once carried a fake gun. They acted out a real fast robbery of the prof. Prof then asked each of us what we had seen. Some concentrated on the gun, some the masks, and others just hid under their desk.

This is why attorneys are allowed cross-examination of a witness to find out the parameters of what the person saw and how they viewed it.

Incidentally, the prof no longer does the fake robbery. One of his students was also a Chicago cop and had the robber collared before the prof could stop it.