Self - Observation:

Backstory on this 'thought',

Wilhem Wundt, founder of the first psychology lab, is often associated with this school of thought despite the fact that it was his student, Edward B Titchener, who coined the term to define the school of psychological thought.

Nature:

Consciousness is INNER EXPERIENCE

EVERY LIVING BEING has this inner experience

Every living being must ALWAYS HAVE HAD this inner experience

THE BEGINING OF THE MENTAL LIFE DATE FROM THE BEGINING OF LIFE

So all psychology must begin with SELF - OBSERVATION..