So now RichardKennaway's comment says that the earliest appearance known to Wikiquote is from 1851 ... but it links to Wikiquote's page about Jonathan Swift, with that same quotation. Richard, did you half-edit your comment?
No, Google led me to a version of the quote in the section of unverified Swift quotes. I didn't notice that the same thought with slightly different wording appears earlier with a verifiable attribution.
Another month, another rationality quotes thread. The rules are: