Sentence:  He asked, how can you truly know the <WSD>"rockness</WSD>" of a rock by categorizing it? Simply dividing it into smaller pieces, weighing it, and looking at its properties may tell you a great deal about what it is like, but what it is, its essence, its <WSD>"rockness</WSD>", escapes such analysis ... 
Meaning of the word marked with <WSD> tags:  The essence of what it means to be a rock; the qualities that make a rock what it is.