Wikipedia calls the novel of ideas a subgenre of philosophical fiction, without defining the first term. The Routledge Encyclopedia of Narrative Theory (2010) uses the terms "novel of ideas" and "philosophical novel" interchangeably. In articles published together in the New York Times in 2016 under the headline "Whatever Happened to the Novel of Ideas?", Benjamin Moser also uses the terms to mean the same thing, and Pankaj Mishra, while eschewing the word "philosophy", draws no distinction between them. Writing in the Financial Times in 2012, Jenny Erdal describes the two forms as "close cousin(s)", but she does not explain the difference.
There does, however, seem to be general agreement that the novel of ideas is quite rare nowadays.
Since not all ideas are philosophical, I do not favour a definition of the novel of ideas as a type of philosophical novel, it seeming that if one is a subtype of the other then the relationship should be the converse. But in that case, what definitions might we use? And if neither wholly includes the other, and therefore each has instances that are not instances of the other, how might we define the two genres as "close cousins"?
Google's Ngram viewer returns the following for the use of the two terms over time:
I have looked briefly at a number of other sources, including encyclopedias. While several define the two terms as equivalent, several others do not. But I have so far found only one source that offers clearly contrasting definitions, namely Leszek Kolek's 1975 PhD thesis. The author defines one of his aims as being to provide "a definition and description of the genre called 'novel of ideas' on the basis of (Aldous) Huxley’s fiction". Achieving his aim, he describes its distinctive features as being found in
- its “material”: that is, explicit ideas appearing in the fictional reality;
- a reduced semantic significance of character and plot;
- a semantic and structural balance between explicit and implicit modes of presentation,
where an explicit idea is "an abstract, syncretic and explicit statement analysed in essayistic utterances of the characters, the narrator’s comments, and in various forms of quotations" (p.106).
He goes on to say that the novel of ideas differs from the philosophical novel "owing to the syncretic character of explicit ideas" and its "lack of a 'thesis'", the implication being that a philosophical novel shares the above three features except that its explicit ideas are non-syncretic and it contains a thesis. (It could also be that he means to suggest that the expression of ideas in a philosophical novel is achieved using means that have a greater proportion of the implicit rather than the explicit, but he does not say so explicitly.) Kolek qualifies the distinction, however, by acknowledging that "we" lack a precise definition of the philosophical novel (p.107).
How far this gets us, I am not sure.