 It seems intuitively obvious that when one takes a sip of water, or adds a bit of water to a glass of water, that one does not have in the glass precisely the same water both before and after. Even if the same amount of water from a different batch replaces the water which was taken away, we still would not have the same exact water before us after the replacement. And, if we perform electrolysis on a batch of water but preserve any matter from being lost we can rightly say we have all the same matter as we did before, just not the same water, since there is none. We can construct similar examples for any type of matter. What can explain and unify these obvious intuitions is that there is a most general category of entity—masses of matter, where all the fundamental parts of any mass are essential to it. Such masses of matter cannot change parts whatsoever.