It is an odd notion to think that someone owns an idea. It's in your head, but if it's to become anything at all, it must go out into the world, but how is it possible to hold onto it? Whatever we mean by ownership, intellectual property can only be a rough analogy to it.
The Constitution gives Congress the right "to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries." The purpose and the scope are altogether clear: "to promote the progress of science and useful arts," with an equally explicit scope: "a limited time." Intellectual property seen in this way seems to mean that, should I write or invent something, I should have an exclusive right to the profits. I do not own these words, nor have I ever, nor do these words belong to anyone; it is only the right to any profit garnered that belongs to me.
Ideas, by their very nature, demand to be free. Everybody knows that James Watt invented the steam engine, right? Wrong. It was an improvement on the Newcomen engine, which was an improvement on the Crawley engine, and so on down the line. So who should properly own the idea, Heron of Alexandria? It's doubtful that even he came up with the idea on his own. Invention is inherently a communal act.
Art too, is communal. No artist creates completely alone, separated from culture, from a world of predecessors and descendants. The artist has his rights, but none of his works can ever be his alone. Take Stravinsky's works. Should all profits go to Bach's descendents because Stravinsky took so many themes, ideas, and inspiration from the master? Should all profits of philosophy go to the descendants of Thales of Miletus?
Certainly, artists have the right to profit from their work, but perpetually? Suppose I made some physical object, let's say a hammer. Once I've sold the hammer, the profit I realize from that sale is all that I'll get out of it. Ideas may have the property of being infinitely reproducible, but should I that mean that I ought to have exclusive rights to them forever? The world too, has its demands. If progress in the sciences occurs one funeral at a time, progress in the arts only really occurs when copyrights expire. Were I to keep the idea for myself, nobody else could build upon it. The recent Supreme Court decision in Eldred v. Ashcroft makes a mockery of the very idea of intellectual property. It allows the extension of existing copyrights for 20 years (from the already obscenely long life plus 50 years), a provision which passed into law just in time to extend Disney's copyright of Mickey Mouse. I ask of you, what, if any, spur to progress in the arts could there be in extending existing copyrights? If anything, it stifles innovation. Rich entertainment corporations, freed from the necessity to invent, could just reuse what they already have. What creative energy do the shareholders of Disney have that merits their profiting from Mickey Mouse?