Starting with Joint Attention, infants learn to follow the eye-gazing or pointing of others by seven to nine months old. They innately understand attention before almost anything else.
As an awareness, I can attend or tend to. My attention breeds awareness: it notices, it observes, it listens, it cares, it organizes, it guides. I can direct my attention inward and outward. My attention seeks an object. When I am the object of my attention, I notice that I am the subject of my attention. I am looking at the eyes in the mirror, but I am not the eyes themselves, and I am not the mirror, nor the image I see in it.
I can direct my attention inward or outward, but attention still needs a target. Whether pointed inward or outward, attention gathers data which allow us to resolve our ignorance and assess changes in our quality. Attention is the input of consciousness' feedback loop.
Inward attention observes, and evaluates the quality of, the self. "I am feeling serene and joyful as I write this to you." My mind looks inward as I attend my emotional state. We build knowledge of self by observing ourselves from various perspectives and by noticing ourselves in various states, conditions, and sets of circumstances.
Others that can attend themselves and others—that are able to direct their attention inward and outward, observe themselves and others from various perspectives, and notice themselves and others in various states, conditions, and sets of circumstances—they are conscious beings. Others that cannot attend themselves or others around them are non-conscious objects, whereas others who possess attention but do not use it are still conscious.