According to the Oxford Dictionary of Quotations, these three observations are from Mark Twain's Notebooks.
As well as 28 books, including the respective adventures of Tom Sawyer and Huckleberry Finn, Mark Twain wrote countless letters, essays, travelogues, and lectures in his lifetime. He also produced between 40 and 50 pocket notebooks, which were edited by his ' literary executor', Albert Bigelow Paine, and first published in 1935.
A sample page, reproduced on the website of Brigham Young University, presents eleven similar observations, beginning with "The human race consists of the damned and the ought–to–be damned" and ending with "The altar cloth of one aeon is the doormat of the next." This is page 346; so who knows how many such pithy remarks are to be found in even the edited version?
© Haydn Thompson 2023