Does knowledge require the impossibility of doubt?

As a beginner in philosophy, I got the impression that philosophy is all about arguments. You put in statements (premises), use some rules of argumentation to manipulate these premises, and reach other statements (conclusions). Is there a way to argue for the rules of argumentation themselves? I mean, we use them all the time but how do we know that they are true? What kind of rules would we use to prove the rules of argumentation? Can we use the same rules? Thanks.

According to statistics one in five people experiences depression. If depression is so common, how do we know it is an illness and not just a normal part of being human?

Suppose someone is thinking about killing himself. Can philosophers or philosophy give him reasons for or against doing it? Or isn't suicide a philosophical subject?

My question is whether or not my disagreement to the next statement makes sense, and what you philosophers think about my argument. There is a popular belief that if aliens were to "visit" us, it would be in a destructive and war-like manner. I disagree. In order for a civilization to evolve and grow to one capable of interstellar travel, it would have to be a culture strong enough and intelligent enough to grow into and support such an advanced race. - If a civilization had the mindset of destroying something like a planet and every intelligent being on it, its society could not develop into something so great as to travel among the stars. - On the other hand, war has given our world many technological advancements in the past. Does this thought process make sense, what are your thoughts concerning my thinking?

Can the proposition, "God is unknowable" be defended? If something is unknowable, how can we know that it is unknowable?

Do Existentialists believe that all actions are permitted because there is no God and no meaning to life?

Science states that space is endless, and ever expanding. But, if we are inside the planet earth, the planet earth is inside the galaxy, the galaxy is inside space, then what is space inside? What is it expanding in? And if space is endless, how can it expand?

Surely Christians have to believe in free will. If they believe in a loving God who controls their destiny and that all bad things happen for an overall good (otherwise God would not let it happen), then wouldn't it be impossible to believe that God would send a person to hell as punishment for something they had done, as it would have been God's will for them to do so in the first place?

Of the many strong arguments against free will, I find the following to be the most convincing. Some theorists have suggested that free will seems to require one to be responsible for the way one is, "mentally speaking." For example, in order to be responsible for my decisions, I must have chosen, in a conscious way, to be in the particular mental state I was in at the time of my choice. Hence, free will seems to require preconsciousness; but this can be pushed into an infinite regress. Do you know of any possible answers concerning this argument?