Evidence Is Seldom Unambiguous
We slide conveniently between the words evidence and proof. When we hold an idea ourselves, it is seldom on the basis of proof, but because we believe the evidence weighs in favor of that idea. Yet we demand proof from those we dispute with. Unless someone produces an argument which compels us to change our minds, we hold to our previous idea. Those we argue with, frustratingly, demand the same from us.
Proof is seldom available. Even in mathematics, the usual gold standard in everyday conversation, proofs are contextual. Change the base, change the surface, and your elegant proof evaporates.
There is evidence for all kinds of propositions, including many that are not true. A person may be innocent of murder even though he owned the murder weapon and hid it, was known to be in the area, and had motive. I don't mean to merely say that "he was found Not Guilty;" he may be actually innocent.
Even when evidence is overwhelming in one direction, it is still usually mixed, and it is foolish to appear not to recognise this. Was communism a good idea? On one side of the scales are the 100-200 million people that communist governments killed, and the poverty of 90% of its citizens. It would take a great deal of evidence in the other direction to overcome that. But there is some evidence -- people put it forward all the time. They supported the arts of high culture, ballet and classical music. They made advances in opthamology. On a deeper level, does not suffering produce great literature? Isn't it worth something that they inspired people to reach for a more perfect world? Yes, these are all good things. They don't make much of a dent in accounting against all those dead people and poor peasants, but it's something.
Evidence can point in several directions at once as well. Say three people are exposed to a disease. One decides to pray, one decides to take medicine, and one decides to just wait it out. A week later, they are all well. This does not prove anyone's position. The evidence would weigh somewhat in favor of the third person -- they were all going to get well in a week no matter what -- but it's not proof by any means. One of the three might have actually caught a separate disease or an additional one; one of the first two may have had a peculiar susceptibility to the disease which should have killed him; the third person's mother may have been praying for him. Each can count the improvement in health as evidence. In none of the cases is it compelling evidence.
When we are trying to convince people of an idea we usually overstate the point for rhetorical effect. That's not contemptible if we recognise that's what we're doing. But it is intellectually dangerous, as we quickly come to believe ourselves. I have an uncle who sends me commentary from the hard left from time to time. "George Bush got us into a pointless war and has alienated the world." As a shorthand way of expressing that you believe that the president's actions were a major reason for military actions which have been a net loss, and have cost us more friends than we've made, the overdramatic statement is only mildly annoying. We can't spend all our time in discussion qualifying every point. But to take the statement as simple fact is just silly. As would be an opposing sentiment that "George Bush has saved America."
In religious or philosophical discussions, I am usually tempted to just not attend to people who make impossible pronouncements. "There's no more evidence for God than there is for the Easter Bunny." Well, there's not much difference in the amount of proof for either, but we've already established that proof isn't really the issue. As to the amount of evidence, the difference is profound. An individual might find the evidence for God insufficient, as there is no evidence that is unambiguously in favor of a deity. But when people try to claim that there is no evidence whatsoever, I suspect that something else is in play. Some need not to believe is ruling the intellect.