Hey folks, I have a question about natural selection: what is it?

No, this is not another evolution vs. creation post (at least, that's not my intent), nor is it me proclaiming I don't believe in NS.
In fact the type of creationism I favor makes heavy use of natural selection to account for the variety within species or groups of species.

Recently however I've been reading another book and it discussed natural selection in a philosophical way, showing that there are multiple ways to define NS and all of them were unscientific.

I found that rather strange, since obviously we can see that some individuals or some species can live on while others go extinct. Since they are not actively selected by an outside intelligence (like man or God or aliens) it would be oke to say that "nature selects".

Now I will not repeat his arguments here, for that may put you in a frame of mind about how to define NS, rahter I'd like you guys to give me your best shot of defining NS and then I'll see if his arguments are still valid when applied to your definitions

One more thing about what I meant with "scientific", here I mean that it is (a theory or hypothesis that is) falsifiable (Popper).