Not sure that he's literally referring to the act of birth. I interpreted it to mean the beginning of a human consciousness.

I think determinism is the most straightforward way to address the idea of multiple possibilities existing at the same time. Saying that anything could have been in a different state is abstract thinking.

The original post is a little vague. Schopenhauer1, what exactly is your stance on these "metaphysics and ethics" you mentioned? Many ethics are based on the notion that one could be another. Does that mean you disagree with all those ethics? Because the reality of what we are is too far from the abstract thinking of what we could be? Or maybe you disagree with the way people describe the justifications of their ethics? What were you getting at?

Yes, I agree that people don't exist before they exist. However, sometimes it is metaphorically useful to speak as if humans somehow exist outside of their current humanity. Doing so can summarize some complex ethical ideas into an easily understandable notion, even if the notion itself shouldn't be taken literally.

For example, let's say a natural disaster strikes another country. Many people are injured/homeless and the country is requesting financial help. You could respond by saying: "Who cares? I don't live there. Not my problem. No thanks, I'll keep my money." Your offended friend might respond by saying: "That's wrong! You could have been born there!"

Technically, no, it was not possible for you have to been born there. The only "you" that exists is the one that (fortunately) avoided the natural disaster. But your friend's point is still valid and logical, even if they used a metaphor. Maybe they didn't even realize that they were speaking metaphorically. Nonetheless, your friend is correct in pointing out that you didn't choose which physical body your consciousness is associated with. And that means you can't take credit for being born into the more fortunate location. Which also means you can't fault the victims for being born into the less-fortunate location. This realization should influence your ethical beliefs. It would make sense to donate to the disaster-struck country, because you would be helping a consciousness that is in the same circumstances as your consciousness; you both had to live where you ended up being born. And you aren't sure exactly why you are you and they are them. So maybe that is you. And if they are you, it would certainly make sense to help yourself out of a tough situation.

I don't think the definition of "everything" can include the illogical. That would be like using the lack of a definition to create a new definition. In order to understand an idea, that idea must have some sort of logic to it. Illogical things are nonexistent to the human mind. Logic is the only language our brains can comprehend.