Whilst we may obtain great knowledge about the world, there is another dimension which has been largely lost in our modern society. This dimension is related to one’s ‘inner life’, which remains dormant from childhood, but which can be reawakened and undergo a process of evolution. This is what Gurdjieff called ‘essence’, in other words, who we really are. Essence is related to personality, but distinct from it, and the inner evolution of essence is the real meaning of ‘know thyself’.
P.D. Ouspensky relates a lecture given by Gurdjieff in relation to the ancient maxim ‘Know Thyself’:
The next lecture began precisely with the words: ‘Know thyself’. “These words,” said Gurdjieff, “which are generally ascribed to Socrates, actually lie at the basis of many systems and schools far more ancient than the Socratic.”
From P.D. Ouspensky, In Search of the Miraculous, Fragments of an Unknown Teaching.