"The consciousness of 'myself' and the distinction between 'myself' and all other selves, the antithesis between 'myself,' the observer, and the external world, the observed," Barfield wrote early on (in History in English Words), "is such an obvious and early fact of experience to every one of us, such a fundamental starting point of our life as conscious beings, that it really requires a sort of training of the imagination to be able to conceive of any different kind of consciousness. Yet we can see from the history of our words that this form of experience, so far from being eternal, is quite a recent achievement of the human spirit" (HEW 164). No insight is more central to Barfield's thought than this one--formulated with striking clarity before he was thirty.

The self-consciousness we now take as common sense is, in fact, the product of the entire evolution of consciousness.1

See in particular "Personality and Reason" (HEW 156-77), "Self and Reality" (RM 155-75), "Review of Julian Jaynes' The Origin of Consciousness."
1Barfield has no patience for the argument--proferred by linguistic analysts--that we have always been self-conscious but have merely not had the language to articulate our awareness of it. In "Philology and the Incarnation" he gives it the back of his hand.
    It simply does not make sense to say that at one time self-consciousness was an existing fact which had not yet been discovered. You can be unaware of many things, but you cannot be unaware of being aware. In this case the discovery and the birth of the thing discovered are one and the same event. (RM 233)