Another potential theme: religion? But the religion is science, and Watney's faith that it would pull him through, as in his determination to "science the shit out of" any problem..;-)

Hi. I can't find the exact quote of the final speech Watney gives to his students. He says something about how big problems may seem impossible to solve, but you just solve a small problem, then another small problem and solving these small problems will eventually lead you to solve the larger problem. It does relate in part to what you already mentioned about the indomitable human spirit. But I think it also has to do with how we deal with problems that may seem difficult or even impossible. Breaking them down into little, easier-to-solve problems will get us closer to the goal.

I found part of the speech and an interesting article that says more about this final speech far better than what I wrote. : http://thefederalist.com/2015/10/03/what-the-martian-tells-us-about-life-on-earth/ "“at some point, everything’s going to go South on you. You’re going to say, ‘This is it. This is how I end.’ Now, you can either accept that, or you can get to work.” You have to solve one problem, Watney explains, and then solve the next problem, and then solve the next problem, and “if you solve enough problems, you get to go home.” ... a descripton of life itself"