As a means of critiquing the limits of rationality, the inseparability of existence and essence is fundamental to Heidegger’s reproach of humanism in general and, in particular, Sartre’s perpetuation of the separation of existence and essence through existential humanism, which Heidegger contends is irrational since it contains metaphysical baggage. Though Heidegger traces the problem of humanism to Plato and the subsequent history of ontology that requires destruktion, the fact that Heidegger highlights the rational ontologies of Kant, Descartes, and Aristotle means that these three have exponentially skewed things away from the question of the meaning of Being—this question, as such, is the focal point of Heidegger’s notion of rationality. Yet, I would argue that Descartes figures more central to Heidegger’s destruktion than the other two, since Heidegger’s phenomenological method of destruktion—which is most notably apparent in the inseparability of existence and essence—according to Heidegger’s outline in Being and Time, depends on:
The ontological foundation of Descartes’ ‘cogito sum,’ and how the medieval ontology has been taken over into the problematic of the ‘res cogitans’.
As an “ontological foundation,” the Cartesian cogito—more accurate as cogito ergo sum than cogito sum—sets up a relation between reason and subjectivity through dualism, separating the acts of thinking from existing, and essence from existence respectively. This distinction rests in the Cartesian use of “ergo” in cogito ergo sum, which I believe operates analogously to “precedes” in the Platonic essence precedes existence. Heidegger seems to make note of this, but also rightly recognizes that “medieval ontology has been taken over” by Descartes cogito ergo sum. Although I find it unclear what Heidegger means by “medieval ontology,” Descartes definitely introduces a phenomenological problem with his cogito—this problem is rooted in two phenomenologies on either side of the ergo. Modern ontology, as Heidegger cites in The Basic Problems of Phenomenology lecture, appropriates cogito ergo sum to differentiate between res extensa (being of nature) and res cogitans (being of mind) under ways of being (122-125)—the former aligns with existence, and the latter with essence. I would argue that, for Heidegger, the “problematic”—or die Problematik—of the res cogitans is not exclusively in the purposeful separation of it from res extensa, but, through a critique of Sartre’s humanistic assertion of existence precedes essence, the impulse to define res extensa and res cogitans as different substances: the former physical (or material), the latter non-physical (non-material).