Although there are several sub-themes prominent in the novel, there are many examples from the novel that point to one main, overarching theme. I think the theme is self-discovery and coming to terms with who you are without people telling you who you are or what you're supposed to be.
From the beginning, Matt is treated as an animal by everyone around him. Rosa's cruel punishment and constant taunting of calling him an "it" or "beast" largely affected the way he thought of himself. At one point, him and Tam Lin are speaking in the oasis and Tam Lin explains how many people hate Matt and are afraid of him because he's different. He also explains that Matt himself is like a photograph of El Patron at the same age as Matt is then (Farmer 80). These things all contributed to Matt's view of himself throughout the book.
"No one can tell the difference between a clone and a human. That's because there isn't any difference. The idea of clones being inferior is a filthy lie (Farmer 245)." This quote explains that though the book tells you to not let someone else change your opinion of yourself, trusted individuals can help you on your discovery to finding yourself. When El Patron dies, Matt finds out he is human (though still a clone), and proves he really isn't different than anyone else. This shows how he overcomes everyone's opinions and ultimately discovers who he is.
There are a few important sub-themes as well. One of them is your own worst enemy is yourself. This connects to the main theme because Matt often brings himself down and thinks of himself as an animal at the beginning of the book. He allows other's comments and ideas of him to bring him down. For example, after the horrible treatment by Rosa at the beginning of the book, Matt was traumatized and wouldn't speak or interact with humans for a fairly large portion of time. Another important sub-theme, again linked to the main theme, is the importance of friendship. I think this is important because there are several people, such as Tam Lin, Celia, Maria, and his friends from the Plankton Factory, that support Matt and are important to him. The novel showcases those strong ties of friendship and how they influence Matt's life and decisions.
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