 At a time, it is easy to distinguish and identify masses, which in this context are considered Mere Sums. Assume a toy model of a world with only 20 simples, each given a unique number from 1–20 as a name. By UFM, 1–4 will fuse at \(t_1.\) Let us call that sum ‘Quadri.’ Quadri will be distinct from any sums with distinct parts, since no sum can have different parts from itself at a time. Will Quadri be identical with anything ‘else’ composed of the same fundamental parts? Suppose 1 is an electron, and 2–4 are quarks. Suppose 1–4 compose a hydrogen atom at \(t_1.\) Call the atom ‘Hydro’. By the definition of Mere Sums, Quadri’s parts’ proximity to each other is irrelevant to its persisting, but not so for Hydro, which, if it loses its electron, will cease to exist as hydrogen. (The lone proton is a cation of the hydron variety). So Hydro cannot be a Mere Sum. The same will go for any putative object composed of 1–4 which do not have the same persistence conditions as Quadri. And so any fusion resulting in a Mere Sum of 1–4 is Quadri; and 1–4 compose no other Mere Sums than Quadri. And so something is Quadri at \(t_1\) if and only if it has all and only 1–4 as parts at that time. And so all those parts are essential to it at that  time.[19]