Cats were often familiars to workers of magic because to anyone used to wrestling with self-willed, wayward, devious magic--which was what all magic was--it was rather soothing to have all the same qualities wrapped up in a small, furry, generally attractive bundle that...might, if it were in a good mood, sit on your knee and purr. Magic never sat on anybody's knee and purred.
Robin McKinley, Spindle's End
cats, magic, purring