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.