We believe cars are artificial, (designed, built with intent) because we have evidence of people building them, no evidence of nature managing it without human intervention (And remember: Absence of Evidence is Evidence of Absence), and even less of nature developing things that work, look, and function the way we consider cars to function because, barring intermediaries such as ourselves or some other intelligent species, it lacks the ability to do so.
Cars ARE natural, really, it's just that humans are the tools Nature uses to create them. It depends on what sort of "natural" you're using here. Edit:
Like mentioned by Induction.
Rocks are usually 'natural', but can also be artificial. These things are all definitional - we observe certain properties, and stick things in those categories. Most of it isn't based on any real truth but rather on categorical usefulness.
People generally use natural and artificial only when referring to items or classes of items that need to be compared to some other class of items, when there's a strong correlation between "naturalness" or "artificialness" and some other property.
I don't really see where this is going though.
Further, can I ask, does religion or belief in a God stifle science?
Historically? Yes. In many ways - both institutional and individual. It's not inherent - Non-denominational Deist beliefs are pretty unlikely to influence scientific research, for example. But belief in god is only, exceptionally rarely, /just/ a belief in god, because its common causes are not the sort of things that are exclusively limited to "god exists".
And most of the routes people find to god, yes, are least somewhat damaging to science (and rational thought) through their actions on their members and those outside their group, or overtly hostile to science.
The real problem tends not to be a belief in god, which while rarely rational can often be somewhat reasonable, (as in, the action of choosing to believe can be rational even if the belief is not). The problem tends to come more in the form of believing in one particular set of god-granted moral scriptures, which are far far FAR more irrational to believe in than simply the existence of a god, and almost always hostile to science in part or in whole.