An interesting etymology on a word that changed meaning due to changing concepts.
From: English words from Latin and Greek elements.
The word quintessence is an example of this. The ancient Greeks believed that there were four basic substances which composed the universe: earth, air, fire, and water. In addition to these, Aristotle postulated a fifth element, which was so much lighter and subtler than the others that it was difficult to classify and so was termed simply “fifth being.” This name entered our language as quintessence (QUINT-, “fifth,” and essence). In the middle ages the fifth element came to be regarded by the alchemists as the most essential part of any substance. With the advance of scientific knowledge the idea of five elements was discarded, but the word quintessence remained, having lost its older significance of “fifth element” and keeping only the meaning of “the concentrated essence of anything” or, by extension, “the most perfect manifestation of some quality,” as in the expression, “He is the quintessence of conceit.”