Principle of individuation

To us readers in the 21st century, it seems like Schopenhauer is endlessly and tirelessly pointing out the extremely obvious.

What is the Principium Individuationis? Broadly speaking, Schopenhauer is talking about the Latin term used by the scholastics for, you can guess, the principle of individuation.

The question is, how can we differentiate between one object and the next? What is it that separates one thing from another? In other words, how can we conceptualize multiplicity in the world? Keep the word multiplicity in mind, because it's important.

It's simply the state of there being multiple things in the world.