Nowadays the situation is almost reversed. Eye witness testimony is regarded with great suspicion, to the extent that it is now unlikely that a trial would even go ahead if there was only eye-witness testimony with no other collaborating evidence.
When I speak to theists the issue of 'witness' is never far from the surface. If we are debating religion - as I am inclined to do - then it is almost a certainty that, at some point, the theist will resort to an argument along the lines of 'well what I saw/experienced/felt was enough to convince me that God exists'.
I am not limiting this to religion, however. Religious-type beliefs usually rely on a similar 'conviction' on the part of the believer. This applies to conspiracy theorists, alien abductees, spiritualists, and a host of other belief systems for which there is little or no empirical evidence.
If we experience something then we are reluctant to even consider the possibility that it didn't really happen as we remember. Our only link with external reality is via our senses. If we can't rely on our senses then what CAN we rely on?
Nevertheless, the evidence is conclusive and the matter is beyond doubt - our senses are constantly creating perceptions, and memories, of things which did not happen, or did not happen as we recall them. This is not a matter for debate - the evidence is clear and conclusive.
In this essay I will show just a few examples of how your senses can deceive you.
The problem starts with perception. Consider the eyes - sight is our most important sense, if we measure by the amount of processing resource each sense occupies in the brain. There is still much work being done, and much more to do, with regard to the processing of sense perceptions, but we know that a significant amount of our total capacity is used in processing visual information. Nor should this surprise us if you consider that the eye delivers an upside-down image (like a pinhole camera), which is shaky and, worse, has a thumping great hole in it (where the optic nerve enters/exits the eye). The brain has to do significant amounts of processing simply to present us with an image that doesn't have an obvious hole and which is the 'right' way up.
In fact there isn't enough raw processing power in the brain to do this methodically in real time (consider, more than 12 images per second, each image consisting of about 150 million signals from the rods and cones in the eye - all this has to be combined, inverted, treated and delivered to our consciousness as an uninterrupted stream of vision). The brain simply doesn't have the raw processing power to do this.
In order to see, our brain and nervous system use a number of 'non algorithmic' approaches - in simple terms we use short-cuts. Much of the time this causes no problem - evolution demanded that our vision should allow us to spot potential predators and potential food as a priority, and for most of the time our vision is adequate to these tasks. Sometimes, however, the short-cuts mean that we see isn't what is actually there. This is true even when we KNOW that to be the case. One way of illustrating this is the use of 'optical illusions'.
One of the most startling visual illusions is shown below. This illusion is so convincing that I have had pupils almost in tears with frustration, refusing to accept that their eyes were deceiving them.
This is an effect generally known as the checker shadow illusion.
What you (should) see is a chequered board on which is placed a green cylinder. The cylinder is casting a shadow, diagonally across the chequered pattern. Two squares are highlighted - A and B. B is in the area covered by the shadow whilst A is outside that area.
Here is the startling fact - squares A and B are exactly the same colour. Our eyes tell us that A is much darker than B, but this is wrong. Our brain works-out that the shadow is present and automatically raises the brightness of everything in the area covered by the shadow. This is entirely unconscious, so even now that you know they are the same colour it is highly unlikely that you actually SEE them the same. You continue to 'think' that A is darker than B.
(A true sceptic - which is what I am trying to encourage - should, at this point, require more than my simple assertion that they are the same colour. Click on the 'evidence' button to satisfy your scepticism).