The idea of a fully articulated philosophy of the novel does not really get going until Georg Lukács wrote Theory of the Novel during World War One, though it was not published until 1921 by which time Lukács’ political world view had changed. There may be some large scale work on the philosophy of the novel I have missed before Lukács, but there is nothing which has lasted as a point of reference.
Of course there is important work on the philosophy of the novel before Lukács in remarks by Friedrich Schlegel (as well as other Romantic ironists), G.W.F. Hegel, and F.W.J. Schelling. Going back further there is some work which indirectly addresses the novel. The New Science of Giambattista Vico is the most obviously relevant since he gives great importance to epic, particularly those attributed to Homer. Not only are there ways in which the novel is the continuation of the epic that give Vico relevance: the way in which Vico places the epic in the context of a transition from heroic-aristocratic world to legal-democratic world sets up thinking about the novel and might have been influenced by the early modern evolution of the novel as a literary form.
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