I think, therefore I am.
Rene Descartes

Descartes argued that consciousness is axiomatic because you cannot logically deny your minds existence at the same time as using your mind to do the denying. However, his formulation and derivation of the axiom were wrong, in that he assumed one can be aware, without something to be aware of.

Consciousness is the faculty that perceives that which exists.

Directly or indirectly, every phenomenon of consciousness is derived from one's awareness of the external world. Some object, i.e., some content, is involved in every state of awareness. Extrospection is a process of cognition directed outward -- a process of apprehending some existent(s) of the external world. Introspection is a process of cognition directed inward -- a process of apprehending one's own psychological actions in regard to some existent(s) of the external world, such actions as thinking, feeling, reminiscing, etc. ... A content-less state of consciousness is a contradiction in terms.
Ayn Rand, Introduction to Objectivist Epistemology

Because to be conscious is to perceive something, consciousness requires something outside of itself in order to function; consciousness requires and is dependent upon, existence.

Further, a consciousness cannot merely be conscious of itself, as Descartes implied. To be a consciousness, it must be conscious of something external to itself. Only after it is conscious of something external can it identify itself. Like a car motor that generates electricity for it's own use, it needs to be kick-started by something outside of it. It needs existence.