It wasn’t Twain who wrote that letter: that quip was posthumously attributed to him, and have been used by multiple others. The earliest source we have for it

It wasn’t Twain who wrote that letter: that quip was posthumously attributed to him, and have been used by multiple others. The earliest source we have for it is a letter by Blaise Pascal, some 250 before Twain ever thought about writing letters, or anything else for that matter.