I don’t think that what you are saying about belief and doubt logically follows from the flaws of perception. One simply works around human limitations and one accepts that these limitations are always there whether talking about knowledge or belief.
Well if belief is acceptance that their perception is true, in the face of all these flaws, why persist in accepting it as true? More than just perception is required for knowledge, but not belief. Keep in mind, knowledge is not merely accepting something as true.
How can we test our perceptions?
I think it is both: an understanding problem and an ideological problem. And this seeming unsurmountability would be solvable, if it was not a mix of both: an understanding problem and an ideological problem.
Probably you remember the follwing conversation:
Again: Ignore him?
During my study at the university I have met many types of students who were back then exactly like he is now. It is their ideological conceitedness that makes them so cocksure and ignorant, so that they do not only appear like stupid people but really are stupid people. You do not really have to care whether their incapacity is based on genetic defects or on ideological defects, because the effect is the same old stupidity as ever.
If I see something, I accept it as belief or knowledge based on previous experience. I have calibrated my thinking by comparing it to other people and I have a reasonable belief that my brain and senses are operating normally.
Therefore, when I’m in my house and I see a cup of coffee next to me, I accept it as knowledge that there is a cup of coffee there. I don’t doubt it. (Should I consider it belief rather than knowledge? An interesting question.)
If someone comes in and tells me that there is no cup of coffee there, then I would reevaluate the situation. It it possible that I am wrong? Is it possible that he is wrong? Is he trying to prank me?
If my perceptions are out sync with other people many times, then I would suspect that I’m malfunctioning. 
It’s not necessarily a matter of perceptions being out of sync with others either though, being the problem. Illusions are susceptible to all people, for one example
If all people have the same illusion, then you need not worry about it. It can be factored out.
If some people have some illusions at some times, then you need to look at what many people are experiencing and by comparing and contrasting, you can come to objective conclusions as to what is reasonable.
Visual Perceptions can are testable through, well, touching for one example. For example, perhaps when you leave for work one morning, there is what looks like a piece of steel rebar pierced through your windshield of your car. The glass looks shattered. But then you touch it, the rebar is no longer rebar, it is styrofoam upon touching it. The glass shattered on your windshield is no longer glass, nor is it shattered, but it is a sticker that is put on top of a glass to give it the impression that it is shattered. When you look closer, from different angels, your visual perception realizes that the styrofoam isn’t even pierced through your windshield, your cognitive processes gave rise to you to see what probably was a shadow on the inside of your car to make you think that it actually pierced through the windshield. So usually perception is often tested with other perception and reasoning and logic are applied to test your perception - which coalesces with your prior knowledge and experience to result in a more accurate perception. But is it ever complete? Your perception can be peer reviewed for further assurance that your faculties aren’t. If you look again, do you perceive the same? Perceptions are often validated through prior knowledge and understanding. But does that make it knowledge when we look at something and identify it conceptually in our minds a certain way, at first glance, second glass, even through touching?
Each object usually requires different “tests” to verify that you know what you are seeing… Is it something that is consistently there, day in day out, saw by everyone? Such as the sun? Do we know the sun exists? Yes, of course we know the sun exists. There’s implications aside from it being seen, that verify its existence. Heat comes from the sun, so we can feel the heat when its rays touch us. Life thrives from the sun, chlorophyll forms from chemical interaction with the sun. We see this through growing plants, growing gardens, testing its implications. We could know the sun exists, even if we never saw it.
“Factored out”? There comes a possible dilemma, that how do we know what is an illusion, if all people have the same illusions…
Yes there are ways to confirm certain perceptions are illusions, such as we have with say the “water on blacktop” illusion we often see when we drive… but what if we don’t know of an illusion, we haven’t reasoned it as one…
phyllo
How are you defining “see” here, phyllo? With your eyes or with your mind as in coming to understand it?
If with your own eyes, why accept it as a belief - belief is accepting something because of certain things which you do see but not enough to be able to prove it…not being certain.
If i see a tree, I know it’s a tree. That’s knowledge. That’s memory. Why is it necessary to compare what you see to what others see unless you know that you sometimes hallucinate.
I would probably use the word “fact” here rather than knowledge. But knowledge works. Knowledge is the sum of everything which you see around you that proves that cup is there. Hmmmm 
Seeing the cup of coffee is not belief. You either know a thing or you believe a thing.
Again, only if you are prone to hallucinations would you doubt yourself. Hallucinations deal more with beliefs than with true knowledge. Have you drank from the cup of coffee yet?
But why would you do that? Wouldn’t it depend on WHO you are and know yourself to be? Maybe the people you hang out with take hallucinagens (sp?)
Is knowledge also a belief?
It probably doesn’t mean much but the world “belie” is within the word belief - which means of an appearance) fail to give a true notion or impression of (something); disguise or contradict.
All knowledge begins as belief until examined and proven within reason.
If everybody has it, then it’s not an illusion, it’s the way things are. It becomes irrelevant. Apply Occam’s Razor and remove concern about such things from your thought process.
I disagree Phyllo, if everyone is under the impression of an illusion and everyone does not know it, its still an illusion. Perhaps its irrelevant, so much as long as it remains an illusion. I don’t want to apply Occam’s razor here and believe the simplest reason, because I don’t need to believe.
To still call it an illusion requires a God’s-eye-view, which we don’t have. Therefore, it’s impractical to call it an illusion.
So some of what you think you know, might be an illusion.
Would it then be false knowledge you have?
IOW from where you are now, you cannot know for sure that some parts of what you consider known are correct - unless you think the idea of revision, such as we have in science, is incorrect.
This means that some of your knowledge, perhaps just one thing, may in fact be incorrect.
And this is why looking at knowledge as justified beliefs is a good model, because it allows for knowledge to be degraded back into merely a belief that is no longer well justified, even though it seemed to be before.
Where as the binary knowledge vs. belief does not allow for this, since knowledge is true, period and has nothing to do with beliefs.
There is such a thing as collective hallucinations and mass hysteria. People are willing to believe anything for the sake of believing. One peron sees something and then suddenly it becomes like a virus in the mind. Something takes over and what was illusion becomes “real” or collective hallucations.
If everyone has it, it CAN be illusions unless we’re all looking at the same tree…
Your eyes detect energy and pass an energy map to the brain. The brain interprets that as objects. Therefore, “seeing an object” means that it has already processed by your brain.
I would classify anything that I experience directly as knowledge but maybe I’m not being rigorous enough. Maybe, strictly speaking, a lone personal experience does not have sufficient justification. Maybe it should be classified as belief until confirmed. Not sure.
Things that I hear described by others I put into the category of belief until confirmed (or refuted).
Maybe I Imagined drinking from the cup. 
Yeah that’s a factor. I would not trust the judgement of a psychopath or a compulsive liar. In these philosophical discussions, ‘other people’ means other normal, intelligent, functioning, rational people.
That’s what we have been saying to WW3_angry … without much success.
ANGRY
I may not be completely understanding what you’re saying here but if I am - THAT is the nature of belief when it comes to particular individuals who feel the need to simply accept as truth something which has flaws and can’t be proven. For them, the fact that they believe in it gives the truth of it though they’re wrong.
The scientific mind will not do that. The scientist for instance does come from a point of belief at first. You have to believe in order to continue on with the investigation…there is some knowledge there pointing the way. You have to "believe’ in the possibility of something perhaps being true.
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To perceive something to me means to see it in a certain way. You’re right but I also think that more than just perception is also required for belief but I may be wrong. When I look up at the sky, I perceive it. When I begin to think of the “possiblity” of there being a creator god who made all of this, it isn’t only based on what I see at the moment but on other things I know so I think more than perception is required for even belief - though I stop at wondering and go no further. I do not believe. Maybe none of that made any sense. But I somehow intuit that more than perception is necessary for belief. Take the scientist for instance. He perceives what he does but he also interprets that perception a particular way, based on knowledge which he does have then he continues on. Maybe I’m wrong…
You mean knowledge which has been tried and is true.
With all of the words in the English language, you would think that we would capable of find the right ones to explain our meaning. We need more words. lol
But no one is deliberately fooling anyone. The mind creates the ilusion and it becomes belief and that belief spreads to others and it becomes mass hysterial.
Our so-called miracles are an example of that. We need to believe that god is there, that god is with us and has not abandoned us. We need answers. We need to believe in a god in the first place. So. we see something and it becomes misinterpreted because our need refuses to see it for what it is or what it might be. In other words, what we see is all there is and not even that, what we see is transformed into Illusions. They work because there is something in them that we desire or need.
That’s the nature of belief which hasn’t been even examined. Belief which has been examined and held to be more than possible/right/reasonable I think is what Plato was talking about - justified belief. But not certainty.
Possible reasons : confirmation bias / peer pressure / emotional comfort / lack of critical thinking / genuine ignorance