These words were inscribed above the entrance to the temple of Apollo at Delphi, site of the sacred oracle. In ancient Greece, people would visit the Oracle hoping to find out what destiny had in store for them or what course of action to take in a particular situation. It is likely that most visitors read those words as they entered the building without realizing that they pointed to a deeper truth than anything the oracle could possibly tell them. They may not have realized either that, not matter how great a revelation or how accurate the information they received, it would ultimately prove to be of no avail, would not save them from further unhappiness and self created suffering if they failed to find the truth that is concealed in those words -- Know Thyself.
What those words imply is this: Before you ask any other question, first ask the most fundamental question of your life, Who Am I?
Unconscious people--and many remain unconscious, trapped in the egos throughout their lives--will quickly tell you who they are: their name, their occupation, their personal history, the shape or state of their body, and whatever else they identify with.
Others may appear to be more evolved because they think of themselves as an immortal soul or divine spirit. But do they really know themselves, or have they just added some spiritual sounding concepts to the content of their mind?
Knowing yourself goes far deeper than the adoption of a set of ideas or beliefs. Spiritual ideas and beliefs may at best be helpful pointers, but in themselves they rarely have the power to dislodge the more firmly established core concept is who you think you are, which are art of the conditioning of the human mind.
Knowing yourself deeply has nothing to do with whatever ideas are floating around in your mind. Knowing yourself is to rooted in Being, instead of lost in your your mind.
Eckhart Tolle, A New Earth