Consciousness is a difficult question because it is poorly defined and is the subjective experience of the entity experiencing it. Because an individual experiences their own consciousness directly, that experience is always richer and more compelling than the perception of consciousness in any other entity; your own consciousness always seem more “real” and richer than the would-be consciousness of another entity.

Because the experience of consciousness is subjective, we can never “know for sure” that an entity is actually experiencing consciousness. However there must be certain computational functions that must be accomplished for consciousness to be experienced. I am not attempting to discuss all computational functions that are necessary, just a first step at enumerating some of them and considering implications.

First an entity must have a “self detector”; a pattern recognition computation structure which it uses to recognizes its own state of being an entity and of being the same entity over time. If an entity is unable to recognize itself as an entity, then it can't be conscious that it is an entity. To rephrase Descartes, "I perceive myself to be an entity, therefore I am an entity."  It is possible to be an entity and not perceive that one is an entity. This happens in humans but rarely. Other computation structures may be necessary also, but without an ability to recognize itself as an entity an entity cannot be conscious.

Related articles: Nonperson predicates, Zombies! Zombies?, & many more.

ETA: This argument appears to be a rehash of the Chinese room, which I had previously thought had nothing to do with consciousness, only intelligence. I nonetheless find this one instructive in that it makes certain things explicit which the Chinese room seems to gloss over.

ETA2: I think I may have made a mistake in this post. That mistake was in realizing what ontology functionalism would imply, and thinking that ontology too weird to be true. An argument from incredulity, essentially. Double oops.

Consciousness belongs to a class of topics I think of as my 'sore teeth.' I find myself thinking about them all the time: in the middle of bathing, running, cooking. I keep thinking about consciousness because no matter how much I read on the subject, I find I am still confused.