Once, there was a Japanese monk who had a little personal superstition.
Every time he travelled to a new location, he’d find some wood that grew there and make it into a staff to defend himself from any bandits or ne'er-do-wells who attacked him.
He was convinced that the staff, being more in tune with his surroundings, would serve him better in a fight. One day, he explained this to a scholarly friend, who decided to do some investigating.
The scholar started swapping the monk’s staves while he was asleep. Some days, the monk would be using a staff he thought was from where he was, but wasn’t; some days he’d believe it was from elsewhere, when in fact it was the correct staff for where he was; and some days belief and truth would match.
Interestingly, the scholar discovered that it was the monk's belief that mattered - whichever staff he was using, if he thought it matched his surroundings he’d do a little better, and if he thought it didn’t he’d do a little worse.
Of course, since then there have been many more rigorous studies, but that scholar’s treatise remains one of the most important works in shaping human understanding of the place-bo effect.