Moses Herzog's biggest struggle is with his own mind. And, frankly, at times we'd bet that ol' Herz would rather battle some White Walkers north of the Wall than do battle inside his brainpan.
A chain reaction of tough breaks has led the poor guy to wonder why he should bother to go on living. Throughout this book, he thinks about the philosophical basis for existing and even writes a letter to the long-dead philosopher Friedrich Nietzsche, saying,
Dear Herr Nietzsche […] You speak of the power of the Dionysian spirit to endure the sight of the Terrible, the Questionable, to allow itself the luxury of Destruction, to witness Decomposition, Hideousness, Evil. (9.21)
Whoa, bro. Calm down. Take a walk. Take a bath. Put down the paper and stop writing to dead people. Herzog talks like this a whole lot in this book, often to his dead buddy bud-buds, but sometimes just for yuks. He's on philosophical overdrive.
But in the end, it's only by thinking more about his immediate surroundings (and people who are actually alive) that he manages to get out his own head and pull out of his philosophical death-spiral.