This dissertation articulates and defends a view about linguistic competence called 'variabilism'. According to variabilism, the epistemic demands of full linguistic competence vary in a particular way. More specifically, variabilism holds that different individual lexical application conditions (individually essential metaphysical ways of being providing essence-giving satisfaction conditions on reference) vary in whether comprehensional awareness of them is required to be fully linguistically competent with the words whose application conditions they are. For some individual conditions, like those for 'bachelor' (e.g., being unmarried, being male, being an adult), linguistic competence intuitively appears to demand robust awareness of them, while for others, like those for 'water' (e.g., being a chemical, being two atomic parts hydrogen, being one atomic part oxygen) no such demand seems in force. This dissertation seeks to defend the reality of this variability by explaining its objective basis. This defense is developed with the challenge of semantic externalism in mind in particular, a widely embraced view which many regard to render implausible all accounts of linguistic competence which place uniform, robust, and psychologically "internal" epistemic demands on users of an expression. Such a view would seemingly threaten variabilism's affirmation of any words for which linguistic competence requires robust awareness of any of their application conditions. The dissertation concludes by suggesting a handful of potential implications and applications of the theory of linguistic competence developed in relation to various familiar and important topics in the philosophy of language. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]