Trajectory as Substantiated Nothing
ELI5
When something moves, it is never fully "here" — it is always also leaving where it was and arriving somewhere it isn't yet. Sartre calls the path of that movement a "substantiated nothing" because it is real enough to measure, but its whole existence depends on a kind of built-in self-absence.
Definition
In Sartre's ontology of motion, being-in-itself is characterized by radical self-identity—it simply is what it is, without negation, lack, or temporal extension. Yet motion, as a contingent fact about things in the world, cannot be derived from this dense, self-enclosed essence. Sartre resolves this by arguing that motion introduces a peculiar ontological structure: exteriority-to-self, a mode of being in which a 'this' is simultaneously itself and its own nothingness. The trajectory is the name for this structure taken in its unity—it is not simply the path a thing traces through space, but the ontological articulation of that very exteriority, the way the moving being is, at each moment, displaced from itself. As a 'substantiated nothing,' the trajectory is both real (it measures and gathers exteriority into a single being) and nothing (it is made of the negation of stable self-identity, the 'not-yet-here' and 'no-longer-there' of a position). It is, in this sense, the genesis of space enacted in time.
This structure can only be apprehended by a For-itself—consciousness—through temporal ekstasis, because only the For-itself has the capacity to hold past and future together in a unified, nihilating act of comprehension. The trajectory is therefore not a feature of the world available to a neutral observer; it requires the constitutive act of a consciousness that is itself structured by non-coincidence with itself. The concept thus names the point at which Sartre's analysis of motion rejoins his general ontology of the For-itself: just as consciousness is a 'being that is what it is not and is not what it is,' the trajectory is a being whose unity consists in its own constitutive non-presence to itself.
Place in the corpus
The concept lives in the ontological analysis of motion within jean-paul-sartre-hazel-barnes-being-and-nothingness, functioning as a local specification of Sartre's broader account of exteriority-to-self. It is an extension of the concept of Consciousness (as the For-itself) insofar as the trajectory can only be constituted as a unity by a consciousness capable of temporal ekstasis — the For-itself's characteristic non-coincidence with itself is what makes the 'substantiated nothing' legible as a single entity rather than a scatter of positions. Without a For-itself to gather the exteriority into intelligibility, there is no trajectory, only a brute succession of states.
It also resonates structurally with Gap and Lack as cross-referenced canonical concepts. The trajectory, as substantiated nothing, performs in the domain of spatial ontology what the gap performs in the domain of signification and subjectivity: it is a positive structural opening — a 'nothing' that is nonetheless real and productive — rather than a mere absence. Similarly, it rhymes with Negation and Dialectics: the trajectory enacts a dialectical structure in which a being is simultaneously itself and its own other, held in a unity that is constitutively self-undermining. Unlike the Lacanian gap, however, which is irreducibly tied to the symbolic order and the subject's relation to the Other, the Sartrean trajectory operates at the level of the phenomenology of the perceived world, making it an extension of Phenomenology and Appearance: the trajectory is what appears when exteriority-to-self is taken as a constituted unity in the field of phenomenal experience.
Key formulations
Being and Nothingness: An Essay on Phenomenological Ontology (page unknown)
This nothing which measures and signifies exteriority-to-self is the trajectory, as the constitution of exteriority in the unity of a single being.
The phrase "nothing which measures and signifies" is theoretically loaded because it stages a paradox: nothingness — ordinarily the absence of all determination — is here assigned the positive functions of measuring and signifying, making the trajectory an active, constitutive operator rather than a mere void. The further phrase "constitution of exteriority in the unity of a single being" captures the dialectical core of the concept: unity and exteriority, which ordinarily exclude each other, are here co-produced, making the trajectory the ontological form in which a being holds its own self-displacement together as one.