Intellectual Property is the application of the concept of property to the intellectual. There are different kinds of IP, such as: patents, which apply to applicable ideas, and are enforceable regardless of another person inventing the same thing independently; copyrights, which apply to creative expressions fixed in a medium; trade-marks, which apply to product-names and logo's; and trade-secrets, like the Coca-Cola recipe. The focus here will be on copyrights and to a lesser extent patents. Trade-marks and trade-secrets are not being criticised here as they are fundamentally different from the first two, and pose much fewer problems.
A distinct characteristic of IP is that it is orthogonal to, and thus crosses with, physical property . For example if I buy a book, I do own the physical object (the paper), but I am not free do with it as I please, because someone else still owns the expressions and ideas inside the book. With books this is not so problematic as long as books can only be commercially printed, and this printing involves piecemal costs. Adding a little fee for the author is sensible under those conditions, and enforcement is easy because of the centralized nature of publishers, but as this changes and copying becomes free and can be done at home, both piece-wise selling and enforcement can pose ethical problems, as we will argue. In short the functioning of IP, and thus its justifiability depends on historic circumstances.
Importantly, intellectual creations, once created, are not endangered by a tragedy of the commons when held in common. They rather are endangered when appropriated and turned into property. This happens because of what one could call a Tragedy of the Anti-commons, or more fittingly the Tragedy of the Lost Paradise. Imagine a small forest of trees that, once mature, produce an endless amount of fruits. Pick a pear from a branch, and immediately a new one appears, just as edible as the first. Millions could eat from a single branch. This sounds like paradise, doesn't it ? A problem, however is that the trees don't get there by themselves. They need planting, watering, and such care before they start to bear fruit.
Now, to provide themselves with a living the gardeners of the forest - based on their experience with normal plantations - construct a fence around it, making fruit scarce again, and put a fruit-store in front of it, where the fruits are sold piece-wise: a Paradise Lost. It deprives those who cannot pay for something which could, in essence, be multiplied for free. Thus physical objects and intellectual creations are different to such an extent as to make property and intellectual property two fundamentally different concepts, enabling us to hold different views on the justifiability of property and IP.