‘Naturalizing phenomenology’ by limiting it to the ontology of the sciences is problematic on both metaphysical and phenomenological grounds. While most assessments of the prospects for a ‘naturalized phenomenology’ have focused on approaches based in Husserlian transcendental phenomenology, problems also arise for non-reductive approaches based in Heideggerian existential phenomenology. ‘Heideggerian cognitive science’ faces a dilemma. On the one hand, if it is directly concerned with the nature of subjectivity, and this subjectivity is assumed to be ontologically irreducible to its physical enablers yet still metaphysically dependent on them, then Heideggerian cognitive science will either leave that metaphysical dependence an unexplained instance of supervenience, or ground it in speculation about brute metaphysical laws that have an unclear relationship to the ontology of the sciences. On the other hand, if Heideggerian cognitive science is not directly concerned with the nature of subjectivity, but is instead merely aimed at the development of a Heideggerian phenomenological psychology, then it doesn’t fully address the ontological implications of phenomenology’s transcendental approach, and so, while it might succeed in explaining the realization of psychological phenomena scientifically, the existence of subjectivity will remain inexplicable based on the ontology of the sciences. Neither strategy succeeds in ‘naturalizing phenomenology’: either rejects scientific naturalism, or makes its requirements trivial, while either rejects the transcendental dimension of phenomenology, or fails to address its ontological implications.