What is the source of Portia's speech, "So shines a good deed in a naughty world" in Merchant of Venice Act V Scene 1?
Could it be Philippeans 2:14-15
Portia frequently echoes biblical passages. One alternative source for the passage, interestingly enough, could be in Jewish biblical exegesis.
The full quotation you're referring to is Portia’s analogy in the last scene about the communicable, illuminating power of good works:
‘How far that little candle throws his beams! / So shines a good deed in a naughty world’ (V.i.90-1)
That analogy, in fact, echoes an oft-repeated commentary from Rashi, one of the most famous and respected rabbis and biblical scholars in history. In his exegesis of Numbers 11:17, Rashi explains that Moses’ prophecy and divine connection was not diminished by sharing it with 70 elders of Israel, comparing his gift ‘to a burning candle from which many candles are lit’ (Midrash Rabbah, Num. 11:17).