The challenge to understand consciousness was once called the mind-body problem, but is now known as the hard problem of consciousness, to distinguish it from the “easier” task of accounting for behaviour neurologically. It has beleaguered philosophers and scientists at least since Descartes. The “problem” is that we appear to ourselves as both subject and object. You have a body, but is that body your thinking, feeling, and speaking self? You have experience; but what exactly is that compared to things in the world it is experience of, which includes the body? Language is intimately involved. Since we humans are the only creature known to use grammatical language, it seems that consciousness is a problem specifically for us.
Of course, we imagine that other creatures, and possibly aliens and even machines, could be conscious too. In our language, consciousnes is a noun, so it is easy to imagine it as a sort of ethereal substance, potentially permeating everything. (After all, even physics indulges such abstractions as ‘energy’ and ‘field’.) If consciousness is somehow substantial, then the question of how “it” might interact with traditional matter then seems legitimate, even scientific. It is not, however, because consciousness is not a thing but an activity or relationship—not a noun but a verb.
Other creatures (though not stones) actively respond to stimuli and do not just passively react according to physical laws. Sentience is thus about observed behavior of things, which makes no presumption about their possible experience as subjects. (The very idea of ‘experience’ is a human imposition.) We see that people are sentient, and we also intimately know our own personal experience, so we associate the two. In addition, other people can tell us verbally about their experience, from which we conclude that they must be “conscious.”
Indeed, there is every reason, beginning with our similar physiology, to believe that other people are subjects with an inner life like one’s own. However, consciousness is an ambiguous notion, which for humans implies not just awareness of the world but awareness of one’s awareness. (Or, to keep with behavioral language: sentience concerning one’s sentience.) This reflexivity characterizes some human minds, at least some of the time. It should not be assumed about other creatures, even if they happen to be human.
To return to the issue of language, some English pronouns refer to persons—as speaking and experiencing subjects. These can be singular or plural: I, you, we. English also has pronouns to refer to objects, and to others as though they were objects: it, they, them. It has genders for some of these: he, she, and recent variants. Interestingly, there is no gender for the speaking/feeling subject—except when perceived and referred to as an object. This underlines the fundamental category difference between subject and object, which permeates the problem of consciousness. While gender is a matter of the biological body and certainly colours our consciousness, consciousness presents the uniquely embarrassing problem of having (or being) at a self that is a (gendered) biological body. The mind-body problem is a highly personal form of schizophrenia, expressed linguistically in the tension between first and third “persons.”
Consciousness implies an inner agent, a self, which is the apparent subject for one’s experience of the world (including one’s own and other bodies), and is also presumably responsible for one’s actions. Consciousness involves the troubling relationship to the world, the gendered body, and other beings. It raises the question of what it means to be this particular “one”—a subject that is also an object. Indeed, a subject that is only fully conscious some of the time.