What you cannot deny is experience itself, says Advaita. Raman not a concept, but convenient concept that we have created in the waking state to explain. "That you actually saw me, whether the real me or a dream me, that you saw me that you cannot deny," he adds.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Mantra 5-6.