The great megalithic monuments of Ireland were constructed from 4000-2800 BCE. From Sligo to Dublin are monuments that are today referred to as passage tombs. The people who built them left no written records so we can only speculate why they were built.
Something very simple happens when you answer the question "Who are you?" When you are describing inwardly feeling your "self", you are actually drawing a mental line or boundary. Your self-identity, in other words, depends entirely on where you draw that boundary line.
It is said that when he was near death, Plato had a dream in which he saw himself changed into a swan that flew from tree to tree making it difficult to catch him. Of all the philosophers, Plato is perhaps the most difficult to feel that one has him and his meaning properly netted and secure.
