World War II
WWII is a symbolic event in the protagonist's life, because it wasn't experienced as an artifact of history, but as it really happened. The reality of modern warfare was just being appreciated for the first time, back in WWI, but no one could have expected the progress that only a couple decades could bring. By WWII, there were already nuclear weapons. This symbolic remembrance of history is part of what makes David so compelled to record what happens through journalism.
Journalism as a symbol
A journalist experiences reality and reports on it, helping others to make sense of it. So, David's role as a journalist is to clarify the truth of concerning situations, often by putting himself in harms way to get the critical information. Journalism is a symbol for his quest for understanding, and it is a way that he supports himself, so journalistic truth becomes a symbol for survival, like food or water.
The Great Depression
If World War II represents a kind of grim horror and death awareness in David, then the Great Depression represented another kind of truth. His remembrances of the Great Depression literally represent this: that modern man is not immune to famine and starvation, because the economy is delicate like the natural food cycle is. So in the same way famines happen when rain doesn't come, the Depression represents a lack of provision, and it represents struggle.
The brother as foil
This archetypal relationship is one that siblings will understand well. By being right next to each other, he and his brother slowly clarified their identities against one another, sometimes seeing things the same way, sometimes not, sometimes approaching issues the same way, sometimes not. At the end of the novel, the foil is clear: David considers his brother to be a thoroughly Australian man. The implication is that he himself feels detached from his own culture.
The motif of time
The question of paranoia arises in the plot when David wants to make sense of issues that are beyond his understanding or capabilities to understand. These issues start to converge through motif, and the real battle David suffers becomes clear. Through time, he faces death. Through his relationship to the truth of the past, he earns money for his present day, but why? Time is mysterious, and he is captivated by correctly analyzing what happens throughout time, assembling a kind of chain of cause and effect in his psyche, attempting to understand the nature of his human experience.
