"it is difficult to get a man to understand something
when his salary depends upon his not understanding it."

我不知道怎麼翻的順口,

但中文可以念成:
"很難使一個人了解某件事情, 當他的薪水是根據他對那件事情的不了解."

我個人會想成:
"很難讓他了解那件事當他是被付錢不去了解它."

簡短的英文可以是:
"Do what you are paid to do."
"做你被附錢去做的事情."

或是:
"He is doing what he is paid to do."
"他在做他被付錢去做的事情."