This thesis is focused on the intersections of ontology and politics in the work of Michel Foucault and Jacques Derrida. In particular it concerns the ways in which these two thinkers offer accounts of (ethical, social, political) relations which exceed a traditional dichotomy between transcendentalism and empiricism. Both Derrida and Foucault show universal foundations to originate in an anterior play of differences 'between' the transcendental and empirical. However, as this thesis shows, each thinks this anterior 'medium' of relations in radically incommensurable ways: as differance or aporia in Derrida and as power and problematization in Foucault. As such, each necessarily views the other as failing to account for the ‘true medium’ of relationality and so of its violent effacement and disavowal. This incommensurability, it is argued, results in a polemic between them which is explicit in their competing accounts of Descartes’ Meditations and implicit throughout all of their work. This thesis traces the polemic between Derrida and Foucault across their accounts of subjectivity, ethics and politics. It is argued that in their engagements with each of these fields they employ parallel politicizing strategies which are nevertheless wholly exclusive of one another. The incommensurability between Derrida and Foucault reflects a broader problematic which any political thought affirming its own finitude cannot explicitly recognize. Postfoundational accounts of relationality, it is claimed, violently exclude competing philosophical strategies without the capacity of accounting for this exclusion.