2.021 Objects make up the substance of the world. Therefore they cannot be composite.

Why? Presumably because they are, by definition, something like atoms or monads. The "cannot" is logical. So we still don't know what objects are. Black (p. 61): "It is a basic principle for W. that every combination is contingent (cf. 2.0271 on the ‘configuration’ as mutable). If objects were complex, their existence would be a contingent fact and hence they could not collectively constitute the substance of the world.”