# 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  
In this subsection, we describe version 0.1.3 of the Literary Theme Ontology (LTO) [13]. It is a controlled vocabulary of 2130 unique defined literary themes, hierarchically arranged into a tree structure according to the _is-a_ relation. The maximum depth of the hierarchy is 7 and the maximum path length between any pair of entities is 13. LTO upper-level organization is inspired by a traditional classification system proposed by literary critic [PERSON] [13; 52]. There are four upper-level theme domains: \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Series Title** & **Short Name** & **Original Release** & **No. of Seasons** & **No. of Episodes** \\ \hline Star Trek: The Original Series & TOS & 1966–1969 & 3 & 79 \\ Star Trek: The Animated Series & TAS & 1973–1974 & 2 & 22 \\ Star Trek: The Next, Generation & TNG & 1987–1994 & 7 & 178 \\ Star Trek: Deep Space Nine & DS9 & 1993–1999 & 7 & 177 \\ Star Trek: Voyager & Voyager & 1995–2001 & 7 & 172 \\ Star Trek: Enterprise & Enterprise & 2001–2005 & 4 & 99 \\ Star Trek: Discovery & Discovery & 2017–present & 2 & 29 \\ Star Trek: Shorts & Shorts & 2018–present & 1 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The Star Trek television series overview. The Human  
\begin{tabular}{l p{284.5 pt}} Condition: & Themes pertaining to the inner and outer experiences of individuals be they about private life or pair and group interactions with others around them. \\ Society: & Themes pertaining to individuals involved in persistent social interaction, or a large social group sharing the same geographical or social territory, typically subject to the same political authority and dominant cultural expectations. These are themes about the interactions and patterns of relationships within or between different societies. \\ The Pursuit of  
\begin{tabular}{l p{284.5 pt}} Knowledge: & The Pursuit of \begin{tabular}{l p{284.5 pt}} Knowledge: & Themes pertaining to the expression of a view about how the world of nature operates, and how humans fit in relation to it. Put another way, these are themes about scientific, religious, philosophical, artistic, and humanist views on the nature of reality. \\ Alternate & Themes related to subject matter falling outside of reality as it is presently understood. \\ Reality: & This includes, but is not limited to, science fiction, fantasy, superhero fiction, science fantasy, horror, utopian and dystopian fiction, supernatural fiction as well as combinations thereof. \\ \end{tabular}   
Figure 2 depicts a bird's eye view of the ontology. The abstract theme \"literary thematic entity\" is taken as root class. Each domain is structured as a tree descended from the root with \"the human condition\", \"society\", \"the pursuit of knowledge\", and \"alternate reality\" serving as the top themes of their respective domains. Table 2 provides a summary of the number of classes (i.e., literary themes) in each domain and their heights from the root LTO class. LTO is engineered to fit within the Basic Formal Ontology (BFO) top level ontology class hierarchy [53] in order to facilitate interoperability with other emerging fiction studies ontologies [54; 55; 56; 57; 58]. LTO is meant to cover important, operationally verifiable literary themes that can be expected reoccur in multiple works of fiction [13]. In designing LTO, we strove to make sibling classes mutually exclusive, but not necessarily jointly exhaustive. All literary themes are accompanied with definitions, and references when possible. We appealed to the principle of falsifiability in definition writing. That is to say, a well-defined literary theme will be such that it is possible to appeal to the definition to show it is not featured in a story. Take \"the quest for immortality\" as an example, which is defined as \"A character is tempted by a perceived chance to live on well beyond what is considered to be a normal lifespan\". The theme \"the desire for vengeance\" (Definition: A character seeks retribution over a perceived injury or wrong.)