George M. Searle (1839-1918) and Charles S. Peirce worked together in the Coast Survey and the Harvard Observatory during the decade of 1860: both scientists were assistants of Joseph Winlock, the director of the Observatory. When in 1868 George, a convert to Catholicism, left to enter the Paulist Fathers, he was replaced by his brother Arthur Searle. George was ordained as a priest in 1871, was a lecturer of Mathematics and Astronomy at the Catholic University of America, and became the (...) fourth superior general of his congregation from 1904 to 1909. Among the books he wrote for non-Catholic audiences was Plain Facts for Fair Minds (1895). On the 8th of August of 1895, Peirce found that book in a bookstore and the following day wrote a letter to George Searle developing his strong reservations about the question of the infallibility of the Pope. This letter (L 397) is almost unknown amongst Peirce's scholars. -/- After describing these historical circumstances as a framework, the aim of my paper is to describe Peirce's arguments against papal infallibility presented by George Searle in his book, and the contrast between the genuine scientific attitude and the putative metaphysical notion of absolute truth that is —according to Peirce— behind Searle's defense of infallibility. In this sense, Peirce's fallibilism will be explained with some detail, giving an account also of his practical infallibilism: "The assertion that every assertion but this is fallible, is the only one that is absolutely infallible. But though nothing else is absolutely infallible, many propositions are practically infallible; such as the dicta of conscience" (Minute Logic, CP 2.75, c. 1902). -/- Finally, having in mind the present interest in Peirce's religious ideas it will be suggested that some of Peirce's ideas on infallibility are nearer to contemporary understanding of that issue than Searle's defense. "I would with all my heart join the ancient church of Rome if I could. But your book," —Peirce writes to Searle— "is an awful warning against doing so." -/- . (shrink)