Einstein quote
 What kind of "common"?
Common can mean "occurring, found, or done often; prevalent"; "of a sort or level to be generally expected" (e.g., common courtesy); or "showing a lack of taste and refinement; vulgar" (e.g., a common whore).
I think there's some overlap here, because what's abundant tends to be of lesser value in the marketplace, due to the laws of supply and demand. If someone goes to a lot of trouble to obtain something rare (like a diamond), it's probably because it's of superior quality to the cheaper alternative. For example, book sense requires more work to obtain than common sense, so in order for it to be worth the effort, it must have some advantage over common sense.