substantial individuation

each actually existing particular possesses properties specific to it in a manner completely separate from all others, these properties being realised in each object in a manner specific to the object in question:
two or more existents can be neither indistinguishable in every respect, nor fully dissimilar.
they may not be
—fully indistinguishable, as this is dealing with a single object, not a plurality;
—fully dissimilar, as all objects, by necessity, are alike at the very least in the fact that they exist

if considering two discernibles, either
—each exists while not subsisting within some other essence (two beings), or
—one exists while subsisting within the other essence (one being and its property)

and of the considered, either
—a sufficient explanation exists extrinsically
—or one is the sufficient explanation for each