Consciousness is the most (if not only) subjective experience in existence. There is no scientific definition for it, there is no scientific explanation for it. Consciousness is something you have, you experience, but can’t describe.
Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives.Susan Schneider
Yet, lots of people believe that consciousness is a by-product of the brain. Almost all people believe that other people are conscious. And finally, you believe you were conscious five seconds ago and you will be conscious in five more.
All beliefs based on observations about our real world, but they are all beliefs nonetheless. Maybe our consciousness is merely connected to our brain. Maybe you are the only conscious person in existence, and other people are just very good at faking it. Maybe, just maybe, you were not conscious until now. All you know about your past is your current conscious experience of your memory anyway.
None of these claims are falsifiable. Any conscious experience except your own (right now), is nothing more than a belief. And unless we ever define it in a different way than your subjective experience, it will always be just that: a belief.