 The existence of entities like this could easily explain why there is not precisely the same water through these changes. An unqualified view of this kind would be that nothing whatsoever can change parts—this is called ‘Mereological Essentialism.’ More restricted views hold that at the very least masses of matter cannot change parts. Mereological essentialism has been an extremely popular approach throughout history. (Abelard, Reid, Bishop Butler, Locke, Hume, and Leibniz were all mereological essentialists of some kind.)