 Our more restricted ‘Mereological Essentialism for Masses’ (MEM) seems prima facie consistent with allowing ordinary objects to change parts. MEM seems less controversial than ‘Mereological Essentialism for Objects’ (MEO). MEM seems like something of a truism, and perhaps part of the nature of the category of matter. But let us turn to an argument for MEM on the assumption that there are Mere Sums.