Particles and Identity
In the case of statues and similar objects we can separate the identity of the thing from its substance. Thus the statue is not identical to the piece of bronze that composes it, because the piece but not the statue survives being melted down. The piece is the aggregate of its constituent particles, but the statue is a particular form of that aggregate. For composite objects in general, the object is not identical to the mass of particles that composes it: the constitution relation does not coincide with the identity relation.
But what should we say about the particles themselves? Suppose that electrons are physically basic: can their identity come apart from their constitution? Could we melt down an electron so as to destroy the electron but leave its substance intact? That could not consist in rearranging its constituents, because it has none. We cannot destroy the electron and leave the aggregate of its parts intact, since it is not composed of an aggregate. So it is not clear what it would be for its substance to survive the destruction of the electron itself. We can’t remove the form of the electron and leave the elements that compose that form. You might try saying that we could destroy the electron but leave its single constituent intact. But it is that single constituent, so when the electron goes it goes too. Nor can we suppose that the electron is composed of a substance that does not consist of electrons: for what could that substance be? If it were composed of some other type of particle, we could just repeat the argument for that particle.
For simple objects, then, we cannot produce a statue-type example. The existence of the object and the existence of its substance cannot be pulled apart. At the basic level, constitution is identity. Not everything is composed of something to which it is not identical. So the distinction between identity and constitution is not a deep fact about the universe. It arises only at the level of composite objects.