The Unknown Predator.

The moment you stand on a cliff edge or a high balcony and feel the strange, vertiginous pull toward the void—or the sudden freeze of your legs—that is not a rational thought. It is a reflex. For an arboreal primate, a fall of thirty feet meant death. Your brain calculates the drop in milliseconds, bypassing your conscious mind. It doesn't care if there is a safety net. The fear of falling is the fear of gravity’s final verdict.