All ideas are based upon previous ideas, and all previous ideas are based upon even earlier ideas. The chain of ideas extends backward with no clear beginning. Sometimes people come up with the same idea at the same time and sometimes ideas overlap a great deal without being identical. Drawing clear lines of demarcation between one idea and another, the way you might draw a line in the sand to divide a piece of land, is a problematic task. The fuzzy spectrum of ideas does not easily divide up into neat quantized little pieces of property.
Periodically, abuses of intellectual property law end up in the news. Today the issue being discussed is patent trolls. One might conclude that patent trolls are the result of a broken patent system. I would argue that patent trolls are just the inevitable consequence of trying to divide something which cannot be cleanly divided. As long as you allow ownership of ideas, there are going to be specious arguments that can be made—many which even sound quite reasonable—that one person’s idea is somehow being infringed upon by someone else’s. Ideas on the whole are just not very distinct from one another.
There are new patent reforms on the table right now, some promising, some probably ineffectual. Not surprisingly, the reforms most likely to be effective are the ones that result in less patents being given out in the first place.