To understand this, we divide consciousness into two part \cite{Chalmers_1998}. The first part is the "easy problems of consciousness" which include 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. 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. Qualia in its simplest definition refers to "".
Others argue that qualia and hence the hard problem can never be explained cite.
If we now assume physicalism and that qualia exist, then we can follow that today's neuroscience must be incomplete. Thomas Nagel \cite{Nagel_1974} claims that qualia may need new concepts in neuroscience to be understood. We claim further that qualia is a cornerstone to understand mind.