text: **Explanatory gap**
  
  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] 