The mind–body problem is the problem of how the mind and the body relate. The mind-body problem is
more general than the hard problem of consciousness, since it is the problem of discovering how the mind
and body relate in general, thereby implicating any theoretical framework that broaches the topic. The
hard problem, in contrast, is often construed as a problem uniquely faced by physicalist or materialist
theories of mind.
The philosopher Thomas Nagel posited in his 1974 paper "What Is It Like to Be a Bat?" that experiences
are essentially subjective (accessible only to the individual undergoing them—i.e., felt only by the one
feeling them), while physical states are essentially objective (accessible to multiple individuals). So he
argued we have no idea what it could mean to claim that an essentially subjective state just is an
essentially non-subjective state (i.e., that a felt state is nothing but a functional state). In other words, we
have no idea of what reductivism amounts to.[31] He believes "every subjective phenomenon is
essentially connected with a single point of view, and it seems inevitable that an objective, physical
theory will abandon that point of view."[31]
In 1983, the philosopher Joseph Levine proposed that there is an explanatory gap between our
understanding of the physical world and our understanding of consciousness.[42]
Levine's disputes that conscious states are reducible to neuronal or brain states. He uses the example of
pain (as an example of a conscious state) and its reduction to the firing of c-fibers (a kind of nerve cell).
The difficulty is as follows: even if consciousness is physical, it is not clear which physical states
correspond to which conscious states. The bridges between the two levels of description will be
contingent, rather than necessary. This is significant because in most contexts, relating two scientific
levels of descriptions (such as physics and chemistry) is done with the assurance of necessary
connections between the two theories (for example, chemistry follows with necessity from physics).[43]
Levine illustrates this with a thought experiment: Suppose that humanity were to encounter an alien
species, and suppose it is known that the aliens do not have any c-fibers. Even if one knows this, it is not
obvious that the aliens do not feel pain: that would remain an open question. This is because the fact that
aliens do not have c-fibers does not entail that they do not feel pain (in other words, feelings of pain do
not follow with logical necessity from the firing of c-fibers). Levine thinks such thought experiments
demonstrate an explanatory gap between consciousness and the physical world: even if consciousness is
reducible to physical things, consciousness cannot be explained in terms of physical things, because the
link between physical things and consciousness is a contingent link.[43]
Levine does not think that the explanatory gap means that consciousness is not physical; he is open to the
idea that the explanatory gap is only an epistemological problem for physicalism.[43] In contrast,
Chalmers thinks that the hard problem of consciousness does show that consciousness is not physical.[27]
Philosophical zombies are a thought experiment commonly used in discussions of the hard
problem.[44][45] They are hypothetical beings physically identical to humans but that lack conscious
experience.[46] Philosophers such as Chalmers, Joseph Levine, and Francis Kripke take zombies as
"What Is It Like to Be a Bat?"
Explanatory gap
Philosophical zombies