To understand how the journal that Robinson Crusoe keeps during his stay on the island is a reflection on the genre of the novel we first must understand what a novel is.
A novel is a literary work that consists of narration with the purpose of describing interesting events of a set of characters’ lives which in some cases (like Robinson’s) can serve as inputs for reflections and/or introspection. Some of the characteristic of the novel are: it tells facts (either real or fictional but if they’re fictional they have to seem real and credible for the audience), it describes the environment of where the things that are being narrated or described develop, there has to be a character development and has to be written in prose. Finally, the purpose must be spread the experiences (or ideas) of the author to an audience.
This is exactly what Robinson does with his journal, he creates a sort of autobiographical novel in which he describes sometimes (sometimes not so much) interesting events of his life and serves as a reflection or introspection to him as he is writing it down. Through this journal, not only the reader but also Robinson can look into what his thoughts genuinely are and what they tell about his experiences on the island through his stay in Despair Island and how that affects his mental state while being in it.
But this journal that he keeps also serves us as a look into his daily life. An interesting aspect of this novel (the novel itself) is that Robinson while being the narrator of his story also finds himself being an author at the same time as being the narrator due to be the protagonist of his own autobiographical novel.
But why does he keep this journal in such an obsessive way is one of the questions that pop on our minds as we read the novel. One of the possibilities could be that he does it to try to keep the act of him being within a civilized society and not go crazy due to the fact that he is all alone on the island. That is to say, the same reason why he builds furniture, grows food like they would do on the country and tries to domesticate goats.
Apart from this journal where he records his daily activities because he feels the importance of being self-aware, Crusoe also tells us that he keeps a calendar that shows the sense of self-awareness in more detail.
Religion is also an important topic in the novel, and especially the Presbyterian Doctrine emphasizes the idea that an individual has to “keep and reckoning of the state of his own soul”. This is exactly what Crusoe does with the journal, he keeps track of his own self with the journal. He records in it, in an obsessive manner even, his daily activities without caring if it is important or not as a coping mechanism to keep track of his own identity, of his own self and of his character evolution through the days of his stay.
The protagonist goes through a tough journey of self-discovery and his character develops highly becoming a whole new person by the time the novel ends. When the novel first starts, Crusoe is an ignorant character, which one can objectify that is a good thing as it leaves room for growth and become wiser in love (which he does). As he explores the island and becomes familiar with him, he explores himself and develops his own character by pushing his own limits. He goes from being a lost man who misses home and society but then he grows so used to being on his own and becomes so capable of living by himself without a society backing him that after going back to the land he doesn’t like it and goes back to the island.