The prequel to Plato’s Sophist, the Theaetetus, ends with the unanswered question, what is the logos (discursive account, reasoning) appropriate to knowledge? How can one distinguish it from the logos that lacks knowledge? This dissertation argues that the Sophist, through an inquiry of what the sophist is, is a response to that question. This response consists in three basic claims. First, logos forms the heart of inquiry, that is, the ascent from ignorance to knowledge. That ascent consists in logos repeatedly articulating what one understands at a given moment and then examining that articulation from different perspectives. The dialogue shows how the interlocutors’ initial understanding of the sophist is constantly refuted, refined, challenged, and qualified after being articulated. Second, the cognitive powers of perceiving, judging, and thinking all have the structure of logos, and are presented as stages in the ascent. That is, stage one shows the interlocutors’ perceptions of the sophist; stage two, their judgment of him; and stage three, what they think of him. Each stage gradually approaches knowledge without being identical to it. Finally, this absence of identity suggests that logos is necessary but perhaps insufficient for the ascent to knowledge. The process of inquiry, as shown in the Sophist, gestures towards knowledge as a state of mind that is both internally self-consistent and holds beliefs that allow the knower to be “in touch with” the world (a relation that Plato calls “truth”). Logos is insufficient for knowledge for two reasons. First, while capable of achieving a self-consistent state of mind, it does not guarantee that its results will be true of the world. Nor, moreover, can it replace the personal experience that is equally necessary for knowledge. The dialogue suggests this latter point by concluding with a correct definition (logos) of the sophist that is misunderstood by one of the interlocutors (Theaetetus) due to his lack of experience. These limits of logos suggest that the Sophist presents Plato’s self-critique of both the possibility and desirability of the philosophical dream of grasping the world in its purely “logical” aspects.