Meaning of 'Ful wys is he that kan himselven knowe' in Chaucer's Canterbury Tales

The well-known phrase 'Ful wys is he that kan himselven knowe' was written by Geoffrey Chaucer in his most famous work, The Canterbury Tales.

It appears in 'The Monk's Tale' in the following passage:

Thus starf this worthy myghty Hercules.
Lo, who may truste on Fortune any throwe?
For hym that folweth al this world of prees,
Er he be war, is ofte yleyd ful lowe.
Ful wys is he that kan hymselven knowe.
Beth war, for whan that Fortune list to glose,
Thanne wayteth she her man to overthrowe,
By swich a wey, as he wolde leest suppose.

In modern English, the words translate to what sounds clumsy to our ear 'Full wise is he that can himself know' and might be better phrased as 'He who knows himself is very wise'.

The Monk is describing the death of Hercules who put on a poisoned shirt given to him by an enemy. He then muses on the role of luck and how it can humble even the mightiest warrior.