You aren't in a vacuum, you are a biological organism living in a complex ecosystem. In reality.
https://en.wikipedia.org/wiki/Self-preservation
Some people accept that as a species we each want to live, and this is evidenced not just by a survey, but well studied behavior not just in humans, but in nearly all life.
As such, life is recognized by people who observe reality, to be something that we want. And to exist at all, is something we REQUIRE, logically.
When organized in a hierarchy of needs, it's at the top.
You can survive without water for a time, but you cannot survive if you are dead.
Substitute water for other notions of "right to", and you can see that all of these FIRST require the condition that you remain ALIVE, not dead.
This right to life is by virtue of being human, logically necessary for survival, and for the exercise of any and all other "rights".
So given that it results from evolutionary adaptation, we term it "natural".
I'm really surprised this is being quibbled about. Yes, a natural right doesn't EXIST the way a rock exists, it's a relationship, like mathematics, and in any reasonable system of language we use to talk about these things, it's true.
It's not "ought to be", it's logically MUST BE.
You cannot enjoy a right to an attorney if you are dead, that's not an ought.