Don't throw the baby out with the bathwater
Don't discard something valuable along with something undesirable.
Proverbs are intended to pass on popular wisdom and are frequently expressed as warnings - 'don't count your chickens', 'don't look a gift horse in the mouth' and so on.
Of all the 'don't do that'... proverbs this one seems the easiest to agree with.
To that list of don'ts we can add the odd-sounding 'don't throw the baby out with the bathwater'. Sadly, any discussion of the origin of this proverb has to refer to the nonsensical but apparently immortal email that circulates the Internet 'Life in the 1500s' (or 1600s, as some variants have it). One of the claims in one version of that mail is that "in medieval times" people shared scarce bathwater and by the time that the baby was bathed the water was so murky that the baby was in danger of being thrown out unseen. Complete twaddle, of course.
What is unusual about this phrase is that, quite by chance, the mischievous author of 'Life in the 1500s' hit on a correct date - the proverb did originate in the 1500s. 'Throw the baby out with the bathwater' is a German proverb and the earliest printed reference to it, in Thomas Murner’s satirical work Narrenbeschwörung (Appeal to Fools), dates from 1512. Murner wrote in German of course, but we hardly need a translator as he was good enough to include a woodcut illustrating the proverb. The expression was part of everyday German language from then onward (as 'Schüttet das Kind mit dem Bade aus') but didn't emerge in English until the 19th century. The Scottish philosopher and German scholar Thomas Carlyle was well acquainted with German proverbs and translated it in an essay denouncing slavery entitled Occasional Discourse on the N*gger Question (written in 1849 and published in 1853):
And if true, it is important for us, in reference to this Negro Question and some others. The Germans say, “you must empty-out the bathing-tub, but not the baby along with it.” Fling-out your dirty water with all zeal, and set it careering down the kennels; but try if you can to keep the little child!
Despite going against the establishment view on slavery that was held in his day, Carlyle wasn't quite the freedom fighter we might imagine. His analogy compared the dirty bathwater to slavery (to be discarded) and the 'little child' to the useful service provided by the slave (to be kept). He suggested that "the Black gentleman is born to be a servant and is useful in God's creation only as a servant". What he in fact proposed was that servants should be hired for life and given payment, not kept as slaves.
The proverb, in the form of 'do not empty out the baby with the bath water', was in general use in English from the late 19th century onward.
See also: the list of Proverbs.