WordNet is a large lexical database originally conceived as a model of human semantic memory. While its design loosely follows ontological principles, it was developed by linguists and psychologists without the benefit of input from philosophers. WordNet's unexpected popularity as a tool for Natural Language Processing and Knowledge Engineering revealed the irregularities of the lexicon and the need to clearly distinguish the lexical from the conceptual level consistent with ontological research, as articulated in the work of Nicola Guarino and his colleagues. This paper reviews WordNet's development from a lexicon to a resource that incorporates some ontological principles.