According to the philosopher Jean Paul Sartre (b.1905, d.1980), human self-consciousness, as an absolute subjective consciousness, is Not.  

The Being of human consciousness, what Sartre calls the Being-for-itself, exists as not being anything: as a Nothingness. It is a transparency that knows itself as not being anything that it knows.  

What human consciousness is conscious of, or the objects of consciousness, Sartre calls the Being-in-itself.  

If you examine your own consciousness, you will notice that you are aware of the things in the world, and of things (thoughts, memories) in your mind, but you cannot know your own consciousness as other than being conscious of things.  

It is possible to experience your awareness of being conscious with Buddhist meditation. When you can focus on your breathing to the point where no thoughts appear, then you become aware of your pure awareness, your pure consciousness of existing. This is what the phenomenologists call the pre-reflective consciousness.  

The pre-reflective consciousness is the basis of all mental activity; it is what makes thinking (reflection) possible.  

But because it is pre-reflective, this pure awareness cannot be perceived as an object of consciousness, or through thinking (reflection).  

This is the fundamental paradox of human existence and the inherent source of human suffering. It is the existential ailment that motivated the Buddha to search for a cure.  

The problem is that, because we cannot see (know) directly who we are, we look for our identity in the world of things. It is by means of the world (the material, the ideas, customs, imagination) that we create a sense of self.  

But because this self is constructed from impermanent and imperfect qualities, it is always unstable and always becomes source of suffering.  

Our pure awareness of existence, however, is inherently peaceful and content.  

But how to identify this pure existence when everything that we can be conscious of, everything that we can know, is not what we are.  

Sartre proclaimed human-reality, as “a being that it is what it is not, and is not what it is,” will always be in Bad Faith: always trying to become something it is not.

This is what makes understanding Buddhism and the realization of Enlightenment particularly difficult: the realization of your pure awareness.  

To address this specific problem, the Buddha developed the Not-self Doctrine.