Sartre introduces Being and Nothingness, his single greatest articulation of his existentialist philosophy, as “an essay in phenomenological ontology.” Essentially, it is a study of the consciousness of being. Ontology means the study of being; phenomenological means of or relating to perceptual consciousness.

In the introduction to Being and Nothingness, Sartre details his rejection of Kant’s concept of noumenon. Kant was an idealist, believing that we have no direct way of perceiving the external world and that all we have access to is our ideas of the world, including what our senses tell us. Kant distinguished between phenomena, which are our perceptions of things or how things appear to us, and noumena, which are the things in themselves, which we have no knowledge of. Against Kant, Sartre argues that the appearance of a phenomenon is pure and absolute. The noumenon is not inaccessible—it simply isn’t there. Appearance is the only reality. From this starting point, Sartre contends that the world can be seen as an infinite series of finite appearances. Such a perspective eliminates a number of dualisms, notably the duality that contrasts the inside and outside of an object. What we see is what we get (or, what appears is what we know).

After dispensing with the concept of the noumenon, Sartre outlines the binary distinction that dominates the rest of Being and Nothingness: the distinction between unconscious being (en-soi, being-in-itself) and conscious being (pour-soi, being-for-itself). Being-in-itself is concrete, lacks the ability to change, and is unaware of itself. Being-for-itself is conscious of its own consciousness but is also incomplete. For Sartre, this undefined, nondetermined nature is what defines man. Since the for-itself (like man) lacks a predetermined essence, it is forced to create itself from nothingness. For Sartre, nothingness is the defining characteristic of the for-itself. A tree is a tree and lacks the ability to change or create its being. Man, on the other hand, makes himself by acting in the world. Instead of simply being, as the object-in-itself does, man, as an object-for-itself, must actuate his own being.

Sartre next introduces the related truth that the being-for-itself possesses meaning only through its perpetual foray into the unknown future. In other words, a man is not essentially what one might describe him as now. For example, if he is a teacher, he is not a teacher in the way that a rock, as a being-in-itself, is a rock. In truth, the man is never an essence, no matter how much he strives at self-essentialism. The way he interprets his past and foresees his future is itself a series of choices. As Sartre explains, even if an individual can be said to have a certain physical nature, as a chair does (e.g., “he is six feet tall, and the chair two”), the individual nonetheless projects himself by ascribing meaning to, or taking meaning from, his concrete characteristics and thus negating them. The paradox here is great. The for-itself, desiring to become one within the in-itself, imposes its subjectivity on the other’s objectivity. The for-itself is consciousness, yet the instance this consciousness makes its own being a question, the irreconcilable fissure between the in-itself and the for-itself is affirmed.

Sartre explains that as a conscious being, the for-itself recognizes what it is not: it is not a being-in-itself. Through the awareness of what it is not, the for-itself becomes what it is: a nothingness, wholly free in the world, with a blank canvas on which to create its being. He concludes that the for-itself is the being through which nothingness and lack enter the world, and consequently, the for-itself is itself a lack. The absence it signifies is the absence of the unattainable synthesis of the for-itself and the in-itself. The being-for-itself is defined by its knowledge of being not in-itself. Knowing is its own form of being, even if this knowledge is only of what one is not and cannot be, rather than what one is. The human can never know being as it truly is, for to do that, one would have to be the thing itself. To know a rock, we have to be the rock (and of course, the rock, as a being-in-itself, lacks consciousness). Yet the being-for-itself sees and intuits the world through what is not present. In this way, the being-for-itself, already wholly free, also possesses the power of imagination. Even if absolute beauty (to Sartre, the absolute union of being and consciousness) cannot be apprehended, knowing it through its absence, as in the way one feels the emptiness left by a departed loved one, is its own truth.

Delving into the ways individual beings-for-itself relate to one another, Sartre argues that we, as human beings, can become aware of ourselves only when confronted with the gaze of another. Not until we are aware of being watched do we become aware of our own presence. The gaze of the other is objectifying in the sense that when one views another person building a house, he or she sees that person as simply a house builder. Sartre writes that we perceive ourselves being perceived and come to objectify ourselves in the same way we are being objectified. Thus, the gaze of the other robs us of our inherent freedom and causes us to deprive ourselves of our existence as a being-for-itself and instead learn to falsely self-identify as a being-in-itself.