Long post but Carl Sagan is worth it... (and you can find the PDF of the entire book online if interested.)
Insight, untested and unsupported, is an insufficient guarantee of truth. - Bertrand Russell, Mysticism and Logic (1929)
When we are asked to swear in courts of law that we will tell 'the truth, the whole truth, and nothing but the truth', we are being asked the impossible. It is simply beyond our powers.
Our memories are fallible; even scientific truth is merely an approximation; and we are ignorant about nearly all of the Universe. Nevertheless, a life may depend on our testimony. To swear to tell the truth, the whole truth, and nothing but the truth to the limit of our abilities is a fair request. Without the qualifying phrase, though, it's simply out of touch. But such a qualification, however consonant with human reality, is unacceptable to any legal system. If everyone tells the truth only to a degree determined by individual judgement, then incriminating or awkward facts might be withheld, events shaded, culpability hidden, responsibility evaded, and justice denied. So the law strives for an impossible standard of accuracy, and we do the best we can.
In the jury selection process, the court needs to be reassured that the verdict will be based on evidence. It makes heroic efforts to weed out bias. It is aware of human imperfection. Does the potential juror personally know the district attorney, or the prosecutor, or the defence attorney? What about the judge or the other jurors? Has she formed an opinion about this case not from the facts laid out in court but from pre-trial publicity? Will she assign evidence from police officers greater or lesser weight than evidence from witnesses for the defence? Is she biased against the defendant's ethnic group? Does the potential juror live in the neighbourhood where the crimes were committed, and might that influence her judgement? Does she have a scientific background about matters on which expert witnesses will testify? (This is often a count against her.) Are any of her relatives or close family members employed in law enforcement or criminal law? Has she herself ever had any run-ins with police that might influence her judgement in the trial? Was any close friend or relative ever arrested on a similar charge?
The American system of jurisprudence recognizes a wide range of factors, predispositions, prejudices and experiences that might cloud our judgement, or affect our objectivity, sometimes even without our knowing it. It goes to great, perhaps even extravagant, lengths to safeguard the process of judgement in a criminal trial from the human weaknesses of those who must decide on innocence or guilt. Even then, of course, the process sometimes fails.
Why would we settle for anything less when interrogating the natural world, or when attempting to decide on vital matters of politics, economics, religion and ethics?