Intellectual property is an artificial right granted by the government. Even the existence of patents, trademarks, and copyrights depend on the government.

Edit to clarify: Personal (physical) property rights are considered inherent rights. Outside of any national border, you still own your personal property, because that is a natural right. Governments can enforce property rights, but they don't create them.

Against Intellectual Property[1], a monograph by intellectual property lawyer and libertarian legal theorist Stephen Kinsella, tries to show inconsistencies in pro-IP libertarian and furthermore argues that IP is not a valid form of property, and that it collides with physical property rights.

[1]: http://mises.org/journals/jls/15_2/15_2_1.pdf

Depends on whether you think IP is legitimate property. See the book "Against Intellectual Property" for one side.

More specifically, the GPL is a contract you agree to when you use the product. While strictly speaking it should be a more explicit contract, strictly speaking there's a lot of those kinds of issues with our society.

The GPL is saying "Here's my work, I'll trade it with you on these terms". The aggression would be in breaking those terms.

>More specifically, the GPL is a contract you agree to when you use the product. While strictly speaking it should be a more explicit contract, strictly speaking there's a lot of those kinds of issues with our society.

Wrong. You do not agree to anything by using GPL software. You must abide by the GPL terms upon redistribution to maintain permission granted by the license.

It is not a contract. The differences between license law and contract law are significant. Groklaw covers the issue thoroughly.[1]

>The GPL is saying "Here's my work, I'll trade it with you on these terms". The aggression would be in breaking those terms.

The GPL does not say that. It says "Here is my work. You may redistribute it under these same terms." It has no requirement you give your changes back to the original author. No trading[2] occurs.

[1] http://www.groklaw.net/article.php?story=20031214210634851

[2] Yes, there is the special case of submitting patches upstream which is by convention, not by license or contractual obligation.

You can consider the GPL a contract, but consider the hypothetical situation: I download a GPLed software, implicitly accepting the license/contract. Then I violate it by giving you a copy of the software without making you accept the GPL.

If the GPL is just a contract, then you can now distribute the software as you wish, since you're not bound by such contract. Copyright, on the other hand, doesn't work like that: everyone is bound by default, and you need a license like the GPL to "break free" from copyright's restrictions.

I believe some (e.g. Rothbardian) libertarians only consider title-transfer contracts to be enforceable.