But that is changing the issue. It is still true that people used to believe that the Earth was flat (they would fall of if they went to far) but it was actually round. If you don't like that example, there are lots of examples when someone thinks he knows something and doesn't. So why get hung up on the particular example. It is only a diversion from the issue, isn't it?
Isn't the point that you can believe you know something, but be mistaken and not know what you believe you know, so that certainty and knowledge are not the same? And, so, I can know, but not know that I know (i.e. be certain)?
A ball is round because from the perspective of a person, and in relation to a person the ball is clearly round. If the ball grows to the size of our earth, at any given point it is flat. Only by changing our perspective to one larger yet, to that of the moon, the solar system, or the cosmos does the earth become round. It is a matter of scale rather than of fact. Think of what people are looking for with Pi. If you slice a circle into very thin pie shaped slices, in relation to the diameter the bit of circumference seems nearly at 90 degrees, which is as the earth, from our perspective, seems to be. With a circle a ratio is reached between circumference and diameter. Is it accurate. Is it more accurate than the casual observation that from where I stand everything is flat?
In the time or Frederick the Great, the man who became the head of the Berlin Academy, Maupetuis, was called the earth flattener because he showed with astronomical observations that the earth tended to flatten near the poles. Now, it was obvious to any enlightened mind of the times that the earth was round. Actually it is roundish from a larger point of view, and as flat as ever from the perspecive of those who must live on it. There is a moral to the story.
Truth, no matter how highly held is always most hard pressed in proving reality against what is obvious. It does not matter in most respects whether one believes the earth is round, roundish, or flat. That is not the problem. The problem is the same as always, that truth is a form of human relationship. It can suffer as much as any other form of relationship, and even fail, as when those who do not have enough of truth are cut down by the truth of others.
It does not matter what example you choose. Certainty is the enemy of truth. I think truth is simple enough. We know reality when we can concieve of reality. Truth is the measure of our concepts against reality. If our concept does not conform to reality it is shown to be false. If our concept does conform to reality it is true. So soon as we become certain that our concepts describe reality correctly we give up checking them.
Most of our concepts are handed to us at birth. We must learn a lot, and know a lot before rational, critical thinking becomes possible for us. We cannot possibly begin to check all that we know as fact that is given to us, on faith, as discovered and checked by others. And every day we rely on this given knowledge. How do we know it. We don't know it, but are certain of it, and this presents very severe problems for humanity and society, because when things break down, as they often do, we really don't know how to recreate them. There is not much difference between a divorce and a war, or between a war and carting a computer off to the land fill. Knowledge is an extremely intimate and detail relationship that few of us can be said to have on any subject.