Defining property rights in a tangible physical property(i.e. land, water, stuctures, objects, liquids, and air) sometimes precludes the protection of intellectual property(which include patents, copyrights and tradesecrets). Advocates of that Rothbardian definition consider IP to be an illegitimate form of property because they use the legal system to prevent other individuals from engauging in voluntary acts of exchange and production. This line of thinking is rooted(mistakenly, I believe) in a misunderstanding of the nature of property rights and societal institions. This misunderstanding is informed by a misreading of the history of the free market and the common law tradition which beget it.
Without delving too far into a broader understanding of property rights, it is important to understand how they came to be. They aren't always obvious human rights(no human rights really are) and they don't always interact in an internally consistant manner while allowing for a working market economy.
The modern definition of property rights dates back to Roman Law. Ideas like first possession, that if I choose to claim unowned property it now belongs to me, were first applied in a formal manner there. Roman Law also gave us the idea that land would be owned privately but beaches and bodies of water would be common. This was because the area of a beach, and its contents, was difficult to define due to it's changing nature.
The genius of common law, which picked up such concepts, is that it allows for these rights to evolve through precedent as innovation allows new practices to be adopted. Common law allowed for new legal rights and concepts such as IP to come to being as mass communication and industrialization made them practical and prudent. With a secure patent system in place, British and American industries were allowed to invent and industrialize while communicating openly. Before that time, elaborate efforts to hide inventions and methods of production were common. Leonardo DaVinci, a product of the pre-IP world, was notorious for such secretive practices.
The genious of IP, while it is sometimes flawed, is that it has allowed for a high degree of competitiveness in a global market economy without stifling openness.