They are illustrations of this true proverb: ¹  “A dog returns to its own vomit,” ²  and “A sow, after washing herself, ³  wallows in the mire.” ⁴ 

and if he turned to ashes the cities of Sodom and Gomorrah when he condemned them to destruction, ¹  having appointed ²  them to serve as an example ³  to future generations of the ungodly, ⁴ 

Therefore, dear friends, since you are waiting for ¹  these things, strive to be found ²  at peace, without spot or blemish, when you come into his presence. ³