This is part of my writings based on The Problems of Philosophy book. More details about it are here, Also you can follow the other related posts here.
Is there any certain and definite knowledge that any reasonable person could not doubt it?
For a single person, I would say we can have certain knowledge of our senses and that something exists.
Knowledge of senses
By knowledge of senses, I mean If I'm not blind I can be certain that I see some shapes, colors, etc. This is true for our other senses too, I heard some things, I taste some things, etc.
I'm not talking about what I sense, what the sense comes from (if there is such a thing). I'm talking about the sense itself. If I'm seeing an orange wall, I'm not saying that I can be certain that the wall is orange, Maybe there is sort of glass on my eyes that filters out everything but orange color and the walls real color (if we can even say "real color") is blue. But still, I think I can be certain that "I'm sensing (seeing) an orange color". If I'm feeling hungry I can be certain that I'm feeling hunger. There is not any doubt when I feel hurt and pain. It doesn't make any difference to the feeling of the pain If someone slaps me in the face in the real world or if the slap is done in one of my night dreams.
Because I think there is a direct exposure between I and feelings, and senses.
So this was knowledge of senses which I think I can be certain of and it's not doubtable. Other than the feelings, I can imagine things, I think and make conclusions and etc.
Thinking and imagination I think makes me certain that something exists. I won't necessarily agree with Descartes that "I think therefore I am". I'm not saying that I'm certain that I exist, but I'm saying that I'm certain something exists.
Personal and Public certainty
So these two are the knowledge I think I can be certain of, But what about the "...any reasonable person could not doubt it" part.
Knowledge of senses is true for any person individually, No one can be certain that I'm hungry except myself. About Something exists I can confirm that is a belief that no reasonable person could doubt.
So I think there is a certain knowledge for any reasonable person but it's not shared between them. Every reasonable person has knowledge of his/her senses but his/her own senses, which can't get shared. Also knows something exists but the something that he/she experiences differs from other reasonable persons.
Epistemology and Metaphysics
If these two really are the definite knowledge that a reasonable person can achieve and they are the only ones. On defining episteme and explaining the conditions or state we can say that something exists. We should take these two more seriously as they are the example of which we have certain knowledge.
What's the difference between what something appears to be and what it is?
Appearance only has meaning when you accept there is something other than what you perceive. We can presume and it's possible that what it is, is different from what it appears to us.
Supposing that, What we have directly is appearance and it's probably related to what the thing is. If we can even say that the attributes that we sense as appearance applies to what the thing is. For example, we can't be certain that having color, shape, ... even is really a thing for what the thing is. I don't think we have such a thing as real color, the color is the representation of things surface and something that we add to object.
Is there a real desk at all? If yes how such a thing can be?
We call real desk physical object. The collection of all physical objects is called 'matter'.
summerized from book
We can't be sure about an external thing called "real desk". Also, I think it's good to think how much this question is relative. How are our other beliefs or actions rely on our position regarding this matter?
How to be sure that there is a reality at all? What do we have to know what it is?
As said before, I think that Something exists is a belief that we can be certain. So there is a reality but do we face it directly? Do we have direct experience with it? We don't know and maybe we can't.