Being-With
ELI5
Heidegger's idea of "Being-With" says that being human already includes other people at its core — you don't have to prove other people exist, because existing with them is just part of what you are. Sartre thinks this is a good start but doesn't fully explain how we actually experience and conflict with real, specific others.
Definition
In Sartre's phenomenological ontology, "Being-With" (Mit-Sein) is Heidegger's ontological characterization of human reality as always-already constituted by co-existence with Others. Rather than treating the Other as a problem to be solved epistemologically—that is, as a consciousness whose existence must be inferred or proven—Heidegger relocates the Other at the level of being itself: to be human is structurally to be-with. Sartre acknowledges this as a decisive advance over Hegel's totalizing gesture, in which individual consciousness is subsumed under the Whole and the problem of the Other is dissolved rather than confronted. Heidegger at least insists that the relation to the Other must be grounded in being, not in knowledge or representation.
However, Sartre's critique is that Heidegger's move remains formally unjustified: the transition from ontic co-existence (the empirical fact that I find others around me) to the ontological structure of Mit-Sein (that being-with is constitutive of human reality as such) is asserted rather than demonstrated. For Sartre, this leaves the concrete, conflictual encounter with the Other—the look, shame, the fundamental asymmetry between the for-itself and the other-as-object—unaccounted for by a flat ontological declaration. Being-with thus functions in Sartre's text as a critical threshold concept: it marks the point at which ontology must do the work that epistemology cannot, but also the point where Heidegger's own ontology, in Sartre's view, falls short of the required grounding.
Place in the corpus
The concept appears in the source jean-paul-sartre-hazel-barnes-being-and-nothingness-an-essay-on-phenomenological, embedded in Sartre's critical survey of prior philosophical responses to the problem of the Other. It sits at the intersection of several cross-referenced canonical concepts. Most directly, it engages the problematic of Consciousness: Sartre's whole motivation for evaluating Mit-Sein is to find a ground for the Other that does not reduce to a cognitive or representational relation between two consciousnesses—consciousness must find the Other in being, not in inference. Being-with is positioned as an attempt to short-circuit the solipsistic impasse that haunts any philosophy that makes consciousness primary. It also intersects with Alienation: the Lacanian account of alienation likewise insists that the subject is constituted through a field (the Other, the signifying chain) that precedes and exceeds it—Mit-Sein is, in a sense, the pre-Lacanian ontological gesture toward the same insight, though without the formalization of the vel, the forced choice, or the structural loss. The concept also bears on Authenticity: in Heidegger's own framework, Mit-Sein is inseparable from the question of authentic versus inauthentic existence (das Man), a tension Sartre's critique implicitly inherits. Finally, the critique of Hegel invoked here recalls the register of Dialectics and Abstract: Hegel's "optimism" is characterized as abstract in the Hegelian sense—it presupposes the reconciled Whole without passing through the concrete negativity of individual encounter.
Being-with thus functions in the corpus as a hinge concept: it represents the ontological turn that Sartre both requires and finds insufficient, positioning his own account of the Other (grounded in the look and the concrete for-itself/for-others dyad) as a corrective that Heidegger's formalism cannot provide.
Key formulations
Being and Nothingness: An Essay on Phenomenological Ontology (page unknown)
Being, Heidegger tells us, is the Mit-Sein—that is, 'being-with.' Thus human-reality the characteristic of being is that human-reality is its being with others.
The quote performs the ontological turn in compressed form: by identifying "Being" itself with Mit-Sein, it refuses to treat the Other as an epistemological supplement and instead makes co-existence a structural feature of "human-reality" as such — the hyphenated term "human-reality" (Heidegger/Sartre's rendering of Dasein) signals that this is not an empirical claim about sociality but a claim about what it means to exist at all.