What are the most significant themes in "The Merchant's Tale"?

The Merchant's Tale has many themes in common with the other Canterbury Tales. Here are some of the most important:

• May, the scheming and sexual wife of January poses many questions about the role and nature of women: do we sympathise with her and her desire for a younger beau? Do we think she's a scheming whore? Does she make us feel sorry for January? May also stands comparison to Alison in "The Miller's Tale".
• Marriage - is it a good idea or a bad idea? This is the debate had between Justinus (who argues "against", and whose name means "right" or "just") and Placebo (who argues "for" and whose name means "what you want to hear") within the tale itself: and the marriage of January and May, and, to an extent, that of Pluto and Proserpina (the gods who descend in the garden) only provoke that question again.
• Words and signs. You'll notice that the plot of the tale is full of secret signals - secret handshakes, phony wax keys, and (even when January is blinded) a whole semaphore of frantic waving to organise the culminatory non-verbal event of the tale: the sex between Damien and May. Chaucer is always interested in what power words have and when they aren't needed.
• Religion. January's garden is a parody of the Garden of Eden (Damien being the devil/snake!) - and he quotes at length from the Song of Songs. Blasphemous? Affectionate? Sinful? Open to interpretation.