 One can make a further case that sums’ parts are essential to them over time. Suppose that simples 1–9 exist at \(t_1.\) Assuming UFM, then 1–9 fuse to give us the sum \(S\). At \(t_1,\) also by UFM, there is the fusion of 1–8, which we can call S−. Since S− and S have different parts, by  Leibniz’s Law,  they are distinct. If a sum could lose a part and persist, then it could be possible for the fusion S to come to be made up of different parts. Suppose that only part 9 is destroyed. If MEM is false, then S could come to be composed of exactly what S− is. So, if MEM is false, either (i) S became S−, (ii) S− went out of existence, or (iii) S and S− coincide as distinct sums. But, (i) cannot be true: two things cannot become one (although this has been challenged, see Gallois 1998); (ii) cannot be true—how can a mere mass of matter go out of existence without undergoing any intrinsic change? (for challenges to this see Burke 1994, Markosian 1998a and 2004); (iii) violates a very clear intuition—if you have some stuff, and take some of it away, then you do not have the very same stuff (cf. Jubien 1993 pp. 18–19). So (iii) is false. But then all of (i)–(iii) are false. And so MEM is true. (For another challenge to an argument like the foregoing, see van Inwagen 2009.)