In Romanticism Comes of Age, Barfield offers a western understanding of the concept:

If we take the bare expression, "I am that" [Tat Tvam Asi], we shall probably note a certain difference between the tone in which it must have been uttered long ago by the Eastern Yogi and the tone in which it is uttered today by the Western devotee of imagination. There would be a difference of emphasis. For the Yogi, desirous of advancing further along the path of wisdom, the important thing is, or was, to feel "I am that"--there is indeed such an entity as I myself and I can find it by looking at the outer world. That is his discovery. For the Westerner, on the other hand, as he develops his imagination, the novel experience is to feel "I am that." There was never any doubt of there being an entity called "I," he feels, but the great discovery, the advance in wisdom, is the realization that this "I" is not shut up inside this physical body as if in a kind of box, as he had naturally supposed. No, it is out there in the flower and the stone. "I" am not merely this seer but the seen. I am that. (RCA 39-40)