David Hirschman in Big Think:
In his book “Consciousness Explained,” Tufts University philosopher Daniel Dennett calls human consciousness “just about the last surviving mystery,” explaining that a mystery is something that people don't yet know how to think about. “We do not yet have all the answers to any of the questions of cosmology and particle physics, molecular genetics and evolutionary theory, but we do know how to think about them,” writes Dennett. “With consciousness, however, we are still in a terrible muddle. Consciousness stands alone today as a topic that often leaves even the most sophisticated thinkers tongue-tied and confused. And, as with all of the earlier mysteries, there are many who insist—and hope—that there will never be a demystification of consciousness.”
On a base level, consciousness is the fact of being awake and processing information. Doctors judge people conscious or not depending on their wakefulness and how they respond to external stimuli. But being conscious is also a neurological phenomenon, and it is part of what allows us to exist and understand ourselves in the world.
Dr. Antonio Damasio, a neuroscientist from the University of Southern California who has studied the neurological basis of consciousness for years, tells Big Think that being conscious is a “special quality of mind” that permits us to know both that we exist and that the things around us exist. He differentiates this from the way the mind is able to portray reality to itself merely by encoding sensory information. Rather, consciousness implies subjectivity—a sense of having a self that observes one’s own organism as separate from the world around that organism.
“Many species, many creatures on earth that are very likely to have a mind, but are very unlikely to have a consciousness in the sense that you and I have,” says Damasio. “That is a self that is very robust, that has many, many levels of organization, from simple to complex, and that functions as a sort of witness to what is going on in our organisms. That kind of process is very interesting because I believe that it is made out of the same cloth of mind, but it is an add-on, it was something that was specialized to create what we call the self.”