@FOUND SOUL,
Not on this one; but I once explained the principal of Conservation, or Identity as it is commonly known; though it is an understanding that most of us arrive at usually around the ages of six or seven and is an essential element of all rational thought... I used it, as we all do; but I was rather unconscious of it as many of us are... I had no sooner laid this out than I picked up a book by Piaget where he laid it out in the opening pages... I still find many who do not grasp the notion; but I still feel stupid about laying it out myself, for myself...