What is meant by "al-A'yan" (entities or substances), and what is its meaning when used to describe ritual purity or impurity?

"A'yan" is the plural of "'ayn," and it has several meanings in the Arabic language, including: the sense of sight, a water spring, the sun's disc, a dinar, ready money, a spy, the best part of something, and the very thing itself. "A'yan al-qawm" means the nobles among the people. What is meant by "pure a'yan" and "impure a'yan" are the essences of things that are inherently pure or inherently impure.