I was not talking about statements, i was talking about maximes, certain rules according to which we structure our life. "Life is meaningless" is also not really what I wanted to express in my text.

Assuming that knowledge does not exist, then we can not know about anything, which also means we can not know about whether or not knowledge exists. If it is true that we can't know if knowledge exists, then it must be possible for knowledge to exist. So yes, in this specific case, from the assumption that X does not exist, follows that it is possible that X exists. That's a contradiction, the axiom "knowledge doesn't exist" is wrong and thus, knowledge exists.

I hope that clarified things.

But why should I want to "survive and flourish"? Why should I even want to live? There is no reason for that. Our lives aren't more meaningful if we are better at surviving. I agree with you that evolution is the basis for all of our behavior and if that is true i would also call that the meaning of life, because it is a maxim all other maxims are build upon - but this maxim of surviving and flourishing itself is still meaningless, there is no reason for us to follow it and our life does not become more meaningful if we start "constructing our own meanings".

Exactly, there are only purposes building up on each other. Every maxim can be questioned, you can always ask yourself: Why should I follow this principle, why not act otherwise? That's why at the bottom line, there is no meaning that gives you a manual for live and itself cannot be questioned. So you are right, if every statement builds up on a statement that itself is meaningless, everything is meaningless. And that is exactly my definition of the meaning of life: A maxim that all other maxim are based on, which itself must be meaningless. Why should I be happy, why should I strive for power? There is no reason for that, the meaning of life is nothing but a statement about how people behave.

No, I have shown that it is not possible for knowledge to not exist. There is no way that knowledge doesn't exist because if it didn't, there would be a logical contradiction. If was is possible however, it would also be possible that knowledge doesn't exist and that just cannot be the case. Some knowledge must exist, maybe the only one is the knowledge about the existence of knowledge.

I guess whether or not the word knowledge is useful depends on what is actually knowable. All I know is that knowledge exists.

First of all, I totally agree with you that there are things that can never be known, that can neither be proved nor disproved. I was not arguing that everything can be known but that we know at least about the existence of knowledge.

Now about that "every sentence requires the speaker to know what knowledge is"-part: When I say a statement like "The weather is nice", then I don't necessarily know if the weather is actually nice. Maybe I didn't look outside the window and it is actually raining, maybe some people would hate the weather I like; I totally agree with you here but that is not the point I was trying to make.
When I say: "The weather is nice", I am making a statement about what I think reality to be like, I claim to know that the weather is nice. Even if I say: "Personally, I think that the weather is nice", I still talk about me knowing something; knowing, that in my opinion the weather is nice. And this applies to any statement, any question, any sentence in any language; you always claim to know or not know something, which of course requires you to know what knowledge is.

I don't think there can be a good definition of knowledge, because knowledge doesn't require one. Take a look at a dictionary, a book in which every definition of any word of a language is written. It can be quite helpful if you know don't know a few words; you can explain to yourself that word means using the words you already know. However, if you are a foreigner and don't know any word of the language the dictionary is written in, it won't help you. Every definition of a word is made up by other words whose definitions are also made up by other definitions. What this proves to me is that there must be some basic things, experiences, emotions that us humans automatically, intuitively understand. Every language has a word for love, every language has a word for hate and every language has a word for knowledge. There are some basic building blocks of meaning, that everyone intuitively knows and that are the foundation for every other words definition and knowledge is one, if not the most basic of them. Knowledge and truth are such basic terms that it is impossible to describe them to other people and put up a definition. A person not knowing what knowledge is, cannot know that they don't know what knowledge is. In fact, your question "Do you have a definition of knowledge?" proves that you already know what knowledge is, it translates to "Do you know what knowledge is?", a question, that no one not knowing what knowledge is could ask. When you think about it, every question and every statement, literally every sentence in every language requires the speaker to know what knowledge is. And that is exactly why a definition of knowledge is not only impossible but also unnecessary; everyone who could ever ask what knowledge is, already knows it.

I don't really think you got my point; I am not confusing knowledge with possibility of knowledge, what I was just trying to say was that assuming knowledge didn't exist, we could not know if it exists. Now if that is true, if we couldn't know whether or not knowledge exists, then that means it must be possible for knowledge to exist. And you are absolutely right, just because it is possible that knowledge exist, it doesn't actually have to exist (which is basically the definition of the word "possible"). The whole point of my argument was: From the assumption "Knowledge doesn't exist" follows "It is possible that knowledge exist". That is a logical contradiction and so the axiom must be wrong, knowledge must exist. You are saying yourself that "actually it is impossible to know if knowledge exists". But if your statement is true, how can you know that?

What do you know exists? Knowledge always relies on perception. When you think about it, the only way to gain knowledge is to perceive (If you include thinking and feeling in your definition of perception). It might seem odd to ask but how do we know perception exists? If perception is the only way of achieving knowledge and it didn't exist, we couldn't know anything. This also includes not knowing whether or not perception exists. However, if we don't know whether or not perception doesn't exist, that means it must be possible that perception exist. That of course contradicts the assumption that perception doesn't exist and thus, perception must exist. I believe that everything that is perceived exists in some form. For example, even if I lived in a simulated world, a matrix, what I observe is still an image of reality - in this case ones and zeros on a computer. So, to to get back to your initial question:
I might not be able to know whether or not I dream, what I can know however, is that I think that I am dreaming.

You can do anything you want (if physically possible) yes, but you can’t want anything you want. If every conscious decision is based on your will, what is your will based on? If will was truly free, your will would determine itself, an endless string of will determining will determining will. Thats why there must be something foreign taking influence on what we want, I‘d say it is your life experience, your childhood memories, your social environment etc.

When we observe something, we basically know it - in some form - exists. For example, even if I lived in a simulated world, a matrix, what I observe is still an image of reality - in this case ones and zeros on a computer. If we don't observe a specific object however, there is no way of knowing whether or not it's real. That's why I think that reality is, what is observable - not what is observed but what can be. There simply is no way of knowing whether or not something that is not observed exists and so an observer can but does not need to be necessary for reality.