Every day that the courts are in session, person after person tells lies in the witness box. Each will swear to tell 'the truth, the whole truth, and nothing but the truth,' and the majority will fail miserably to do so. Because the absolute truth is a tricky business to pin down.
Take the case of the eye witness. I don't know how many convictions depend on the evidence of eye witnesses, but it's all too easy to assume that because someone believes they saw something, it happened. Human beings are all too fallible. Let's leave aside optical illusions and take a look at the dangers of memory. Because a witness is not describing what they saw, but what they remember they saw - an entirely different thing. Memory isn't like a video. It is a construct from many different inputs and cannot be relied on to play back an event accurately. Let's take three examples from personal experience.
Last Friday I was watching the TV show Weakest Link, or to be more precise, I was in the room while it was on and was half-watching it. Suddenly a question caught my attention. In a knockout competition of 20 teams, how many matches will be played? The player came back instantly with 38, which was said to be correct. But why? I couldn't understand why it was right, or how she managed to answer instantly. After a heated discussion on Facebook, with friends contributing their own answers, someone bothered (there's scientific endeavour for you) to watch Weakest Link on iPlayer. In fact what Anne Robinson had said was 'In a football season, in a league of 20 teams, that all play one another, once at home and once away, each club plays how many matches?' An entirely different question to the one I remembered hearing. One with an answer of 38. Though it was still impressive that the contestant got the answer so quickly (probably because there are 20 teams in the Premier League).
Let's go back a few years. I got an email from a friend saying 'Don't be such a poser. I saw you walking the dog this afternoon, chatting on your mobile phone so it looked like you were working.' Now just imagine he then saw me kill someone. He would happily (well, perhaps not happily) tell a court that he had seen me commit murder. The only problem was, it wasn't me. I didn't take my phone on dog walks then (this was before I got my iPhone), and I had been in all afternoon, waiting for something to be delivered. The friend had seen someone else with a similar dog and thought he was watching me. As far as his memory of events was concerned, he had seen me in the street at that time. And this was someone who knew me quite well.
On a larger scale, there are the stunning experiments done by the Visual Cognition Lab at the University of Illinois. They have a video that shows a number of students playing basketball in the hallway. (You can see it here.) They show this video to an audience and ask them to count the number of times the ball is bounced on the floor. At the end, the audience is asked if anything unusual happened during the video. The majority say 'No.' Now, in fact, part way through the video, someone in a gorilla suit walks past. This isn't a fast subliminal zip. They stroll across in full view, drawing attention to themselves. But the majority of the audience - and I have seen this done - deny seeing the gorilla. The majority of the people in that room would tell an absolutely incorrect tale of what has happened.
I don't have a lot of experience of law courts. Apart from being in a magistrates' court once to apply for an alcohol licence for an event, my only 'experience' is from TV. But I have the suspicion that much more weight is put on what people say they saw than is justified. Yes, there are circumstances where you are giving something your full attention and you make notes immediately afterwards, where your account may well be good. But otherwise, our unconscious ability to edit and modify memory - yet entirely believe that we saw what we think we saw - makes the eye witness a frighteningly risky proposition for safe legal proceedings.