Borrowing an Idea

Is there any original thought left in this world? Or are we left only to borrow little bits and pieces of ideas from one another to create a sort of collaborative achievement? It seems as though throughout history, humans continuously build on one another's ideas by either developing them into something stronger, or disproving them and paving the way for truth. Whichever the case, it seems as though someone else's thoughts or ideas are the reason that our own ideas are able to flourish into something great. Take for instance post-modernism or even pop literature. Both were a response constructed by previous ideas built from already existing genre's, only expanded.

The question of ownership does come into play though when dealing with inspiration. As we all know, there have been instances in the past in which copyright infringement has led to legal battles between separate parties concerning the use, or abuse, of a medium (be it music, literature, or physical artwork). But how does inspiration from a conversation differ from inspiration from a song? A song is nothing but a sort of conversation isn't it? So how does someone own the right to a conversation? Do they own the rights because it makes them money? At this point the deciding factor between plagiarism and innocent inspiration is if that 'original idea' has allowed someone before you to profit from it financially.

Since it can be argued then that there are no 'original ideas' in our world, does that mean that truly nothing is free? Not even our own thoughts? Hopefully it doesn't get too expensive to voice our own opinions.