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.

In the last segment of his argument, Sartre expands on the for-itself as a being of agency, action, and creation and a being devoid of concrete foundation. To escape its own nothingness, the for-itself strives to absorb the in-itself, or even, in more profane terms, to consume it. Ultimately, however, the in-itself can never be possessed. Just as the for-itself will never realize the union of for-itself and in-itself, neither will it succeed in apprehending or devouring the alien object. Thus, at the summation of Sartre’s polemic, an incredible sense of hopelessness dominates the discussion: I am a nothingness, a lack, dehumanized by the other and deceived even by myself. Yet, as Sartre continually emphasizes, I am free, I am transcendent, I am consciousness, and I make the world. How to reconcile these two ostensibly unreconcilable descriptions of human ontology is a question Sartre does not attempt to definitively answer. This avoidance of reaching a definitive point of philosophic conclusion is in many ways intentional, however, in keeping with both Sartre’s personal style and the existentialist maxim that there are no theories that can make a claim to universality.

As Sartre outlines in the conclusion to his work, perhaps the most essential characteristic of being is its intrinsic absence of differentiation and diversity. Being is complete fullness of existence, a meaningless mass of matter devoid of meaning, consciousness, and knowledge. Consciousness enters the world through the for-itself and with it brings nothingness, negation, and difference to what was once a complete whole of being. Consciousness is what allows the world to exist. Without it, there would be no objects, no trees, no rivers, and no rocks: only being. Consciousness always has intentionality—that is, consciousness is always conscious of something. It thus imposes itself on being-in-itself, making consciousness the burden of the for-itself and of all being. On a similar note, the for-itself at all times depends on the in-itself for its existence. In Sartre’s ontology, consciousness knows what it is only through the knowledge of what it is not. Consciousness knows it is not a being-in-itself and thus knows what it is, a nothingness, a nihilation of being. Yet, to Sartre, despite the fact that the for-itself is nothing, it exists only in its relation to being and thus is its own type of is.

From the beginning of Being and Nothingness, Sartre displays his debt to Nietzsche through his rejection of the notion of any transcendent reality or being that humans can know which might lie behind or beneath the appearances that make up reality. That is, the experience of appearances is reality. Although this does imply an emptiness, Sartre does not see it as a negative truth. Freed of the search for some essential form being, we, as conscious beings (all beings-for-itself), are empowered in knowing that our personal, subjective experience of the world is all the truth there is. We are the ultimate judge of being and nonbeing, truth and falsity.

The key concepts of Sartre’s vision of the world are the being-in-itself and the being-for-itself. One way of understanding how they relate to each other is to think of being-in-itself as another word for object and the being-for-itself as another word for subject. The being-in-itself is something that is defined by its physical characteristics, whereas the subject is defined by consciousness, or nonphysical and nonessentializable attributes. These concepts overlap to a certain degree, since the being-for-itself, or subject, is also possessed of some of the physical self, or some of the attributes of an object or being-in-itself. It thus follows that sometimes a being-for-itself can be harmfully and mistakenly regarded as a being-in-itself.

The interaction of beings possessed of consciousness is a major focus for Sartre, and as he describes a being-for-itself to interact with another being-for-itself, the key concepts are “the gaze” and “the other.” Without question, in Sartre’s view the gaze of the other is alienating. Our awareness of being perceived not only causes us to deny the consciousness and freedom inherent to us but also causes us to recognize those very qualities in our counterpart. Consequently, we are compelled to see the other who looks at us as superior, even if we recognize his gaze as ultimately dehumanizing and objectifying. In response to the gaze of the other, we will assert ourselves as free and conscious and attempt to objectify the individual who objectifies us, thus reversing the relationship. The pattern of relations Sartre describes appears frequently in society. The assertion of freedom and transcendence by one party often results in the repression of those conditions in another. Race-based slavery and the treatment of women by men in patriarchal societies are two obvious examples.

Sartre brings up the ethical implications of the ontological vision set forth in Being in Nothingness only at the end of the work. In later works, notably the famous lecture “The Humanism of Existentialism,” Sartre attempts to outline a philosophy of ethics based on an existentialist study of the nature of being. In short, he argues that values are never objective, as they are created by the choices and actions of free individuals. Herein lies the room for hope that Sartre inserts into a work so full of nothingness and lack: freedom is humanity’s curse as well as its blessing, and what we make of that freedom is our own. In it lies great and indeterminate possibility.