From my article on the ghost-story writer M.R. James:
Even hard-headed skeptics must have moments of uncertainty. There is no rational reason to avoid walking beneath a ladder, only a superstitious one. But who doesn’t think twice about doing it — or breaking a mirror, opening an umbrella indoors, or letting a black cat cross your path? “Everyone can remember a time when he has carefully searched his curtains — and poked in the dark corners of his room before retiring to rest,” wrote James. Better safe than sorry.