Sentence:  A tub of butter, contemplated by him, amounts to a Platonic idea. He understands a leg of mutton in its <WSD>quiddity</WSD>. He stands wondering, amid the commonplace materials of life, like primæval man, with the sun and stars about him. 
Meaning of the word marked with <WSD> tags:  The essence or inherent nature of a person or thing.