According to the philosopher Jean Paul Sartre (b.1905, d.1980), human self-consciousness, as an absolute subjective awareness.  

The Being of human consciousness, what Sartre calls Being-for-itself, exists as not being anything, or as a Nothingness. It is a transparency that knows but cannot know itself as any thing.

Sartre calls Being-in-itself what consciousness is conscious of, or the objects of consciousness. If you examine your own mind, you will notice that you are aware of thoughts, memories, and the senses. You also have a feeling of yourself as a person, or ego.

All that you can be conscious of exists as thoughts in the mind, even the idea of yourself. Sartre calls this the reflective consciousness, or the thinking.

What knows or is aware of the objects of consciousness, of the thinking, Sartre calls the pre-reflective consciousness.

The pre-reflective consciousness is the basis of all mental activity. It is what makes thinking (reflection) possible. I call this pre-reflective awareness the Pure Awareness.

Because it is pre-reflective, however, 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 primordial source of suffering. It is the human condition. At the very source of our being, we are nothing, an emptiness.

Because we cannot see (know) directly who we are, we look to create an identity, a reflection. It is through the world (the material, the ideas, customs, imagination) that we create a sense of self, the ego. This is so that we can exist as something that we can know. 

But because this self is constructed of an ever-changing world, we can never see ourselves clearly. It will always be an unstable entity and a source of suffering. We are constantly striving to be something that we can never be. The self is always foreign to our awareness.

This is what Gautama the Buddha discovered. We crave a world (phenomena) that will always be impermanent, suffering, and not-self (the three marks of existence).

To escape this situation, he devised a method for letting go of the world. But this would not be possible if all we were is conditioned phenomena, just aggregates. There would be no escape.

It is through the Noble Eightfold Path the discover our Unconditioned existence, our escape:

There is, monks, an unborn — unbecome — un-made — unfabricated. If there were not that un-born — unbecome — unmade — unfabricated, there would not be the case that emancipation from the born — become — made — fabricated would be discerned. But precisely because there is an un-born — unbecome — unmade — unfabricated, that emancipation from the born — become — made — fabricated is discerned. (Ud 8.3)

The Unconditioned is the pre-reflective consciousness, our pure awareness. It is what is not the world of things. Because pure awareness is unconditioned, it is not susceptible to the world. It is by nature peaceful and content.

What makes it difficult to realize the unconditioned pure awareness is its absolute subjectivity. Pure awareness is what you are. It is self-evident. You know that you exist. You don’t need a reflection. Realizing your Pure Awareness is Enlightenment.

The Not-self doctrine of the Buddha points to the Unconditioned. The Noble Eightfold Path is how you realize it.