Natural v/s Artificial

A few years ago, one of my cousins brought up the topic of "the boundary between artificial and natural". Her argument (I paraphrase) was something along the following lines: 

A man builds a dam, it is labeled artificial (as in non-natural).
When a beaver builds one, it is natural.
Therefore, the boundary between natural and artificial is bogus.

Here is a cartoon from Abstruse Goose, which captures the same sentiment more graphically.