Sartre divides his theory of existence into two basic categories, which he refers to as "en-soi" (in-itself) and "pour-soi" (for-itself), both of which are derived from his theory of consciousness. En-soi existence is a classification of solid things, that is, everyday objects or substances that exist completely by themselves. This explanation becomes clearer in comparison to the category of pour-soi, which is a being of consciousness that defines itself because it is not something else. Pour-soi recognizes a distance between itself and something that is not itself, a nothingness in between the two, while at the same time being conscious of itself. Being for-itself can only exist through consciousness of an outside object. This act of distance is a state of nothing unique to the consciousness of humans, for through this act of separation, one raises questions and realizes possibilities of what is "not" (i.e. a quality lacking from the situation or environment). For example, should an individual be hungry, they can imagine a future time when they will no longer be hungry. This example can be applied to anything the human mind can imagine. It is the meaning behind Sartre's statement that "the self as not being what it is and being what it is not", because the conscious mind chooses not to accept the reality of what "is", but instead work towards the possibility of what currently "is not".
Sartre uses the example of a friend not being at a caf, stating that although the caf is there, he recognizes his friend's absence, thereby inserting an amount of negativity, or nothingness, to the restaurant. By imagining a lack of something, one distances oneself from the world. This nothingness, or lack, that has been inserted into the caf tableau is actually therefore a projection of the nothingness within oneself. The power of conscious negation in this sense is synonymous with freedom - freedom to imagine the possible and, therefore, freedom to actualize the possibility. The state of being conscious demands the self to constantly choose both belief and action. Sartre's approach that consciousness is free at all times is in direct conflict with Freud's model of the unconscious: consciousness is instead transparent and must make a choice in the here and now, regardless of what has happened in the past. However, this does not mean that the self, as subject, cannot be objectified. The most direct route to this relates to Sartre's example of someone caught looking through a keyhole. Through recognition that another consciousness is regarding the self in an objective manner, whether it be quantitative of the physical self's facticity (such as height, weight, ethnicity, et cetera) or judgmentally (as in assumption of purpose), the self is forced to recognize itself in an equally objective manner. The self in turn defends itself by objectifying the other consciousness.
While humans are able to act upon possibilities to make them a reality, they cannot change their essence. Sartre admits that one can choose a life project - in his case, writing - but that the very act of defining oneself is "bad faith", in that it is either 1)