Full Idea
Your sense of 'natural' is founded on final causes, which is a consideration that appears to me pretty uncertain and unphilosophical.
Gist of Idea
The idea of a final cause is very uncertain and unphilosophical
Source
David Hume (Letters [1739], to Hutcheson 1739)
Book Reference
'British Moralists 1650-1800 Vol. 2', ed/tr. Raphael,D.D. [Hackett 1991], p.109
A Reaction
This is the rejection of Aristotelian teleology by modern science. I agree that the notion of utterly ultimate final cause is worse than 'uncertain' - it is an impossible concept. Nevertheless, I prefer Aristotle to Hume. Nature can teach us lessons.