For a long time, consciousness was considered as unobservable and with that, it was reserved to philosophy where it has been a controversial topic ever since. The advantage of philosophy over science in regard to consciousness is that it can consider something like "experience" wherefore science has no measure of yet. To approach experience, we will use David Chalmers division of consciousness into two part. The first part is the "easy problems of consciousness" which we may eventually be able to explain including the following phenomena: the ability to discriminate, categorize, and react to environmental stimuli; the integration of information by a cognitive system; the reportability of mental states; the ability of a system to access its own internal states; the focus of attention; the deliberate control of behaviour; and the difference between wakefulness and sleep \cite{Chalmers_1998}. On the contrary, the hard problem refers then to the problem of explaining why and how  sentient organisms have qualia \cite{Frankish_2012} or phenomenal experience. 
The difficulty of the hard problem emerge from the conception of qualia. Depending on its 
Qualia is at the cornerstone of physicalism 
Thomas Nagel \cite{Nagel_1974} claims that qualia may need new concepts in neuroscience to be understood. Others argue that qualia and hence the hard problem can never be explained cite. 

Are qualia reducible, physical entities?