The Canterbury Tales: The Miller's Tale
How we cite our quotes:
A man woot litel what him shal bityde.
This man is falle, with his astronomye,
In som woodnesse or in som agonye.
John's story a few lines later about an astronomer who falls into a pit as he's gazing at the stars gives us a clue about why he thinks astronomy, or fortune-telling, might make one insane. The idea is that those who deal in astronomy might spend so much time thinking about it that they neglect the business of day-to-day life, finally falling into a trance from which they can't escape.
'What Nicholay! what, how! what, loke adoun!
Awake, and thenke on Christes passioun!
I crouche thee from elves and fro wightes!'
Therwith the night-spel seyde he anon-rightes
On foure halves of the hous aboute,
And on the threshfold of the dore withoute.
Apparently, John's other theory about why Nicholas might be mad is that he is possessed by elves or evil spirits. For this reason John says the "night spell," a charm thought to protect a house against evil spirits when said at its four corners and threshold in the evening.
And if thou telle it man, thou art forlore;
For this vengeaunce thou shalt han therefore,
That if thou wreye me, thou shalt be wood!
It's not clear here whether Nicholas is implying it's God or himself who will curse John with madness should he betray his counsel. If it's the latter, it's in keeping with Nicholas's wish to portray himself as one who dabbles in the occult arts, like fortune-telling. Perhaps he also wishes to be seen as something of a sorcerer.