You are not, nor should you be, the origin of your own thoughts (any more than you are the origin of your own voice).

I think Stallybrass is saying two things with this statement:
1) No work is “original,” i.e., it exists by itself;
2) Work produced in a field is affected by the work that came before it.

I know that one of the problems I have when doing research is making sure that I have a “new idea.” You don’t want to repeat an already known/common idea; you want to “add something new.” Sometimes this is hard to do because there are only so many things you can say about one piece of work. E.g., in Hamlet, you can argue in so many ways whether or not Hamlet is crazy or sane. The arguments for either side can become “boring” because they’ve been said so many times (in the same ways).

Combining this with point 2 – I think what Stallybrass is saying is that you can’t just come up with a new argument. Your argument is based upon reading what others have said and maybe improving it or building upon it or maybe saying it a new way. I think what makes an argument innovative is how it’s put together (i.e., how it “restates” what others say). E.g., a love story can be “boring”/”unoriginal” because it’s about two people who meet and then fall in love. The end. But why are we interested in revisiting love stories? Why are there so many of them? It’s because of the details of the how and the why and the when and the where get to us. The stories can be generally the same, but the people are different and the details are different and so the experience is different.

I think Stallybrass uses Shakespeare as an example of point 2. Shakespeare is considered to be one of the best writers in literature, but a lot of his works are adapted (or just straight out copied) from other sources – others’ stories, history, etc. So when people do research, they don’t just do it in a vacuum. I think this applies to our research because we have to balance “saying something new” with the fact that the “new” is rooted from the old.