A great many people think they are thinking, when they are merely rearranging their prejudices.

William James (1842–1910) American psychologist and philosopher

(Misattributed)

This phrasing is commonly attributed to James, as well as several others, but does not appear in his writings. It appears to have been first said by Methodist Episcopal Bishop William Fitzjames Oldham (1854–1937) in 1906. The quote was first attributed to James in 1947.

William James did express a similar sentiment in his 1907 lecture collection, Pragmatism, Lecture 5 "Pragmatism and Common Sense," which included the following:

Our minds thus grow in spots; and like grease-spots, the spots spread. But we let them spread as little as possible: we keep unaltered as much of our old knowledge, as many of our old prejudices and beliefs, as we can. We patch and tinker more than we renew. The novelty soaks in; it stains the ancient mass; but it is also tinged by what absorbs it.

For more about the more common wording, see: Quote Origin: People Think They Are Thinking When They Are Merely Rearranging Their Prejudices – Quote Investigator®.