Quintessence. That's what it means:

  '(In Ancient and Medieval philosophy) ether, the fifth and highest essence or element after earth air, water and fire, which was thought to be the constituent matter of the heavenly bodies and latent in all things. [C15 via French from Medieval Latin 'quinta essentia' the fifth essence]

From Collins English Dictionary

...and love.

Pardon the comment noise, but I can't help but highlight that "Fifth Force of Nature" is an excellent name for a band.

stupidity

I always treated stupidity as a fifth element that can bring great and surprising tragedies to humanity.