Writing a PhD thesis, I have come to find, is a lot like building a castle on sand. Not, "castles made of sand," like the Jimi Hendrix song, but actually building a stronghold structure on sand. A castle's purpose is to withstand sieges from outside forces, to stand up to a raging army that wants to pillage the land. This is similar to my thesis, which needs to withstand a barrage of questions from examiners, both internally and externally.
Good castles, as evidenced by those still standing, are built on a solid foundation: a hillside of stone, somewhere high that allows for an excellent vantage point of the surrounding areas, close enough to the shore to disrupt water-based attacks, but far enough away to avoid water-based attacks, etc. No structures that are meant to be lasting are ever built on sand because, as a foundation, it is constantly in flux, constantly shifting. Stand on the beach for any given time and one will find oneself sinking. When trying to lean on sand, it moves away.
This is why no one builds houses or castles on sand: the foundation needs to be constant, unmoving.
My thesis should be built on a strong theoretical foundation, something that can be used as a defense against the rabid hordes of examiners at the gate. Unfortunately, the more I read, the less sure, the more shaky, my foundation seems. My argument, then, begins to fall apart, cracks form in the walls, the roof starts to slant, rain gets in, and so on.
Couple this issue with the sheer amount of time between beginning and ending, and further instability starts to form. Certainly, as I am working now, my theory might hold water, but with the mass of criticism constantly published, the vessel that carries my ideas might have holes poked into it, allowing for my argument to leak out the side, leaving behind a dessicated, empty shell, full of holes. And, of course, my own ideas develop over time, refining and changing the original ideas.
I have found at this point, with my topic shifted innumerable already, that I have begun to patch the holes and cracks. I just hope that by the end of this process, my structure looks like a mighty, impregnable castle, and not like a patchwork lean-to held together by Spackle and duct-tape.