"As a dog returns to his vomit, so a fool repeats his folly" is an aphorism which appears in the Book of Proverbs in the Bible — Proverbs 26:11 (Hebrew: כְּ֭כֶלֶב שָׁ֣ב עַל־קֵאֹ֑ו כְּ֝סִ֗יל שֹׁונֶ֥ה בְאִוַּלְתֹּֽו‎‎ Kəḵeleḇ šāḇ ‘al-qê’ōw; kəsîl, šōwneh ḇə’iwwaltōw.), also partially quoted in the New Testament, 2 Peter 2:22. It means that fools are stubbornly inflexible and this is illustrated with the repulsive simile of the dog that eats its vomit again, even though this may be poisonous. Dogs were considered unclean in Biblical times as they were commonly scavengers of the dead and they appear in the Bible as repugnant creatures, symbolising evil. The reference to vomit indicates excessive indulgence and so also symbolises revulsion.