But I think you can take a 'natural law' perspective to make a reasonable case for distinguishing copyrights/patents/trademarks from the traditional notion of property. I attempted that argument in an old HN thread[1], based on the distinction between conventions that arise 'naturally' from an emergent social context, such as traditional physical property, and those that require top-down design in the form of legislative action. I think one could regard physical property as belonging to the former category and IP as belonging to the second.

[1]: http://news.ycombinator.com/item?id=2284636