But, on the other hand, Uncle Abner said that the person that had took a bull by the tail once had learnt sixty or seventy times as much as a person that hadn’t, and said a person that started in to carry a cat home by the tail was gitting knowledge that was always going to be useful to him, and warn’t ever going to grow dim or doubtful.

Mark Twain (1835–1910) American writer [pseud. of Samuel Clemens]

Tom Sawyer Abroad (1894)

Full text.

Variants sometimes seen:

• The man who sets out to carry a cat by its tail learns something that will always be useful and which never will grow dim or doubtful.
• A man who carries a cat by the tail learns something he can learn in no other way.