# An Ontology-Based Recommender System with an Application to the Star Trek Television Franchise
## 2 Materials and Methods
### A Literary Theme Ontology with Application to Star Trek Television Series Episodes
#### 2.3.2 The Literary Theme Ontology
constitutes another example. By insisting on maximally unambiguous theme definitions, we aim to help bring the conversation of whether a theme is featured in a given work of fiction into the realm of rational argumentation. However, we fully acknowledge that the identification of literary themes in stories will always carry with it a certain element of subjectivity. It is the goal of LTO to minimize the subjective element in theme identification. The individual classes populating LTO at the early stage of development presented in this paper were mainly collected by watching Star Trek TOS, TAS, TNG, and Voyager episodes and recording the themes. We selected Star Trek for building up the ontology on account that the television series are culturally significant and explore a broad range of literary themes relating to the human condition, societal issues, as well as classic science fiction. That said, the ontology is admittedly science fiction oriented. In fact, an earlier version of LTO was used for the purpose of identifying over-represented themes in Star Trek series [59]. The later version of LTO (version 1.0.0) presented in [PERSON] et al. [13] is populated with literary themes derived from a more robust collection of science fiction television series and films, including all the Star Trek television series shown in Table 1 save for _Discovery_ and _Shorts_. LTO was encoded using Web Ontology Language (OWL2) [60] and is made available for download at the project's GitHub repository ([[https://github.com/theme-ontology/theming](https://github.com/theme-ontology/theming)]([https://github.com/theme-ontology/theming](https://github.com/theme-ontology/theming))) under a Creative Commons Attribution 4.0 International license (CC BY 4.0). It has also been made accessible in a structured manner through the R package **stoRy**[12]. This paper uses version 0.1.3 of the ontology, which can be accessed through the like versioned **stoRy** package 0.1.3. Functions for exploring the ontology are described in the package reference manual. For example, the command theme$print() prints summary information for the theme object theme, and the function print_tree takes a theme object as input and prints the corresponding theme together with its descendants in tree format to the console. We encourage non-technical users to explore the current developmental version of the ontology on the Theme Ontology website (Theme Ontology (2019). URL: [[https://www.themeontology.org](https://www.themeontology.org)]([https://www.themeontology.org](https://www.themeontology.org)). (Online; accessed 30 June 2019)). \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Domain Root Theme** & **Domain Color-Code** & **Theme Count** & **Leaf Theme Count** & **Tree Height** \\ \hline the human condition & & 892 & 835 & 6 \\ society & & 387 & 362 & 4 \\ the pursuit of knowledge & & 329 & 308 & 4 \\ alternate reality & & 521 & 484 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Literary Theme Ontology v0.1.3 summary statistics. Figure 2: Literary Theme Ontology v0.1.3 class hierarchy overview. (**A–D**) show “the human condition”, “alternate reality”, “society”, and “the pursuit of knowledge” domain themes to three levels of depth, respectively.