Recent legislation restricting the topics teachers may discuss with students raises the issue of what role the teacher should play in society. We argue that this legislative scrutiny of teachers is symptomatic of an aversion to defining what a teacher is. We argue that, though a reluctance to provide an explicit definition of "teacher" may be warranted for reasons of democracy, academic freedom, and diversity, it also leaves open the possibility of political appropriations of the teacher, some of which may be the antithesis of democracy, academic freedom, and diversity. The role of the teacher thus remains subject to perpetual interrogation and change. A well-theorized and stable definition of the teacher, then, would preclude appropriations of the teacher for political purposes such as cultural assimilation, religious indoctrination, or economic gain. Such a definition would need to be sufficiently broad to be applicable to all teachers in all places, while remaining narrow enough to ward off undemocratic impositions. In other words, the definition must be ontological, structural. We employ the work of Martin Heidegger and Jacques Derrida to evaluate the meaning of "teacher." We first give an example of a legally binding document that provides only negative definitions of the teacher. We then provide an overview of Derrida's theory of iterability and Heidegger's philosophy of language, while utilizing and critiquing an alternative conception of the Derridean teacher offered by Charles Bingham. We argue that the teacher, properly construed, ought to engage in an explicit, intersubjective inquiry into the ontological foundations of existence.