I saw this one go by and thought, “Are you kidding me? Do we really want to think that the best Shakespeare could come up with is something like ‘scum of the heart’?” It didn’t help that there’s even places on the net where people asked “What play is this from?” and were told “Merchant of Venice.” Not true. Shakespeare does use the word “scum” four times in his work, but never in this context.
This one took me awhile to find. It is quoted often in the history books, and always with the word “Boston” next to it. For awhile I thought that had something to do with a collection of papers or a particular essay that was being cited. Then it dawned on me that this is an actual person — Thomas Boston, a Scottish church leader born in 1676 (so, not too long after Shakespeare). Here, from Google Books, is the man’s own words:
Observe your hearts all times but especially under temptation. Temptation is a fire that brings up the scum of the vile heart: Do you carefully mark the first risings of corruption.