  It is asserted that consciousness functionally is a vision. Biologically it
is a sensation of work of a brain that converts external and internal signals
into visual output. However, as we well know, human consciousness includes
meaning (interpretation), and therefore functionally possesses an additional
level of complexity. It consists of concepts which are its minimal components
(elementary constituents). A concept is an abstract (i.e. non-existent in
nature) common property of different observables, as perceived by humans. It is
an irreducible entity since it cannot be divided into any other functional
components. At the biological and physical level each concept is a unique
sensation encoded in a nervous system as a sensory and visual image. Concepts
evolve throughout one's lifetime. We continuously create and acquire new
concepts, develop and expand the existing ones, and eliminate and abandon some
others. 'I' ('self') is one of the concepts. To acquire a concept one has to
learn (to teach one's nervous system) feeling or sensing it, and to connect it
to existing concepts, if any. We make a conjecture that both the creation and
the acquisition of a concept is a phase transition. The ability to generate and
acquire concepts is unique to humans and is the only principal difference
between human and animal. We offer scenarios for how this ability could have
been acquired by our predecessors, thus making them humans. It is suggested
that the animal's brain, being a processor of visual signals that converts them
into internal images, has developed this ability as a result of "guessing" or
imagining details that are missing in the actual visible picture.
