OK, first unlink television from knowledge. Quintessence simply means the very best most pure something can be. If you find one politician that does everything right, hand-shakes with the best, supports his constituents and does the most good he has political quintessence. If you are the best in your school at cup stacking (as an example) and your skills are such that other cup stackers are in awe of you; you would have cup-stacking quintessence--or to use the adjectival form you would be the quintessential cup-stacker.
Now as to your television show "WITCH" since they are using the classical four elements of magick and ancient philosophy--earth, air, water, fire--I can only assume they are using quintessence incorrectly as a synonym for Aether. The supposed fifth element that controls emotion and naturally the one thing the ancients knew that was definitely not earth, air, fire or water and that is electricity or in it's raw form, lightning.
Now turn off your t.v. and go read a book. I suggest something from Albert Camus or Tad Williams.