Sentence:  But nonetheless the purpleness of the imagined purple cow will almost certainly be meaner, more <WSD>diaphanous</WSD>, more fleeting than any real-life purple that you ever saw: to imagine a purple cow is just not the same thing as to have a purple sensation (or at least a purple sensation worth the name). 
Meaning of the word marked with <WSD> tags:  Transparent or translucent; allowing light to pass through; capable of being seen through.