An important landmark in the philosophy of consciousness is “What’s it like to be a bat?” 

Thomas Nagel’s 1974 paper.

Here's a video description of the issues by Jeffrey Kaplan.

And a pen-portrait of Thomas Nagel.

 

In his paper Nagel aims to show that physicalism, the proposition that the world is comprised entirely of physical stuff and that there is nothing else, is false. He argues that subjective experience, e.g. experience of the colour red, is something that is in the world but is not physical, i.e. not fully describable in terms of the interaction of light of a particular wavelength with human eyes and brains. His claim is that the experience of 'red' is subjectively interior, and thus not part of the objective, shared, external domain that is subject to scientific or reductive explanation.

 

Nagel's paper led to a split in the study of consciousness between those who want to find an explanation for consciousness in terms of brain mechanics, and those who, feeling such explanation to be impossible in principle, point to a ‘hard problem of consciousness’ that is forever beyond the remit of physical investigation.