Pons asinorum

Pons asinorum is a Latin phrase that means "bridge of donkeys." It is used in mathematics and philosophy to refer to a difficult problem or concept that must be understood before one can proceed. The phrase comes from the ancient Greek philosopher Euclid, who wrote about a difficult theorem that he said was "the bridge of donkeys" for students trying to advance to a higher level of knowledge. In other words, it was a hard problem you had to solve before you could learn more.