You think that you will die just because everyone dies. And you would like to know if you are immortal. How can you know if you are immortal or not?
Inductive inference. All humans have died so far, therefore (in all likelihood) all humans die at some point. You are human, I take it, so there you go.
I take it the questioner is aware that inductive inference is what leads most people to think that they will die. But there's the problem of induction, mentioned in the comments, stated by Hume - which makes it questionable whether my inference that I will die has the status of knowledge.
Inductive reasoning is premised on the thought that the past resembles the future. But why do we think the that the past will resemble the future in this case? Because it always has done in the past. Inductive inference has to presuppose what it sets out to establish and is therefore circular.
You might say that this is as good as it gets - and is how we claim to know most things, therefore I know that I will die. But, on a stricter definition of knowledge, this lack of certainty prevents us from claiming knowledge of our own mortality.
The standard definition of knowledge is justified true belief. I can justify my belief that I am mortal via inductive reasoning but can I can't establish its truth that way - the conclusion that I am mortal still requires verification, and when that happens, I probably won't exist to experience it for myself. If I were immortal, on the other hand, there would always remain open the possibility that I might die at some point in the future, say, at the age of 325. (Though at the age of 150, I might begin to think the evidence is beginning to point in favour of my immortality.) So it seems that one's own mortality can be neither verified nor falsified by that person. Together with the circularity of inductive reasoning, this implies that my own justified inference that I will die doesn't have the status of knowledge and cannot have that status in my lifetime.
Taking this question to a level above the simple "we have seen everybody dies, therefore we are going to die", it's definitely not a stupid question.
- What if we are living in a dream and therefore we live as long as we want to (and then move to something else)?
- What if the world is created because of us and we are God?
There a lot of similar questions that are not that obvious to an open mind.
The answer is: we might be, we might not be. We don't know for sure that we are going to die. We assume we are going to die;
To be honest, there's very little we can be sure about this reality.
You can't. When you're alive, the closest you can approach to knowing you're capable of death is by being very near death, like split second. Passed that, you can't know because you don't have a mind that CAN know. You're already dead.
Before anything can be said of a "knowledge of death", we must, if briefly say what makes a "knowledge" of something. To know, is to create the mental image of a sensory stimulus. In other words, for us to know something, our brain must be able to take sensory input of something, and relying on the intuitions of logic and causality, to establish a working structure and relation with other inputs and previous "knowledge".
What we "know" of the world is the mental imprint of nerve stimuli which creates the image of a living, breathing, temporal "world". For there to be a world, it has to first exist within our own minds.
Therefore, and according to the former points, the only real knowledge of death is of its physical manifestation (ie of the nerve stimuli which simulate and hence create the condtion "dead"). Consequently, one is led to a dual conclusion:
a. The physical manifestation of death is present and is, factual. The nerve stimuli that I receive from around me do confirm that a living being has ceased its living state and is therefore now "dead".
b. The aforementioned nerve stimulus still remains a nerve stimulus, ie, I know death "happens" within the frame of my mental construct, but as for the thing in itself, it is doubtful. I do not know what really exists past my mental construct of it.
The world exists, but it exists within your mind first and foremost.
Some people say that you can reach conclusions about whether some specific event will happen by induction. They are wrong. Induction is impossible. No knowledge has even been created by induction. Nor will any knowledge ever be created by induction. Explanations do not follow from observations in any sense. Nor do observations prove any idea. Nor can any observation make any idea one jot more probable. Inductivism is just another variety of justificationism: the idea that it is possible and desirable to prove ideas true or probably true. Justificationism is wrong. In reality, you can't prove any position or show it is probable. Any argument requires premises and rules of inference and it doesn't prove (or make probable) those premises or rules of inference. If you're going to say they're self evident then you are acting in a dogmatic manner that will prevent you from spotting some mistakes. If you don't say they are self evident then you would have to prove those premises and rules of inference by another argument that would bring up a similar problem with respect to its premises and rules of inference.
In reality all knowledge is created by conjecture and criticism. You notice a problem with your current ideas, propose solutions, criticise the solutions until only one is left and then find a new problem. Experiments are useful only as criticism. Ideas can't be derived from experiment any more than from any other set of premises. Rather, the idea is that you work out how the consequences of one theory differ from those of another. Then you conjecture ideas about experimental setups that would enable you to see the relevant consequences and criticise them. Once you have a setup that works about as well as you can make it work you use it to do the test. If the results are compatible with one theory and not the others then you may have successfully refuted some false ideas. Sometimes a purported successful experimental test will be successfully criticised because a test is a conjecture about something that happened and that conjecture may be wrong, so experiments don't prove anything.
Your knowledge is all guesswork and the only way of sorting those guesses is by criticism. How would that work in this specific case: trying to work out whether you will die. Lots of people have died in the past, but it doesn't follow from this that you will die. You have to explain why they died and whether it is possible to do anything to stop it. People died in the past because their bodies are biological machines and their machine wore out - this is the biological basis of ageing. If we can understand how the human body wears out then it might be possible to maintain it and so people might only die as a result of accidents. Some people are working on how to prevent ageing. Will you die? I don't know. Nor does anybody else, it depends on what problems we solve.
In fact, you cannot give a definition what your being is, what is it to be 'I'. Therefore, without any clear and exact definition you can neither reason about it, nor prove anything - logically from axioms or by inference from whatever experience - your or others.
One can say that to be 'I' is just to live now and here, so the only prove is to run living all the moments.