Most artists are brought to their vocation when their own nascent gifts are awakened by the work of a master. That is to say, most artists are converted to art by art itself. Finding one's voice isn't just an emptying and purifying oneself of the words of others but an adopting and embracing of filiations, communities, and discourses. Inspiration could be called inhaling the memory of an act never experienced. Invention, it must be humbly admitted, does not consist in creating out of void but out of chaos. Any artist knows these truths, no matter how deeply he or she submerges that knowing. (link)
But Lethem also makes a compelling case that modern copyright law needs to be rethought in certain ways. (There are certain echoes of Lessig's arguments in Free Culture here.)
What Lethem doesn't really address is the real sense of violation that people who consume plagiarized works of art feel when they discover that a book or a piece of music is not what it says it is. There should be more scope for free appropriation than there currently is, especially in this digital era, but I think the limits of that appropriation need to be respected. The concept of authorship may have grown distorted with the advent of what Lethem calls "monopoly of use," but authorship is still there (along with plagiarism as its nemesis), and not just in the minds of literary and cultural critics.