What is the significance of this quote to the plot of the play? "A devil, a born devil, on whose nature / Nurture can never stick; on whom my pains, Humanley taken , all,all lost, quite lost! / and as with age his body uglier grows, / so his mind cankers.

I think that Prospero is referring to Caliban as "a devil". Propspero claims he has tried to help the monster but he only grows uglier and more poisonous. Prospero knows Caliban's plan to have the mortals murder him. Certainly the relationship between Prospero and Caliban is at play here. Prospero, like masters of slaves in any era, feels he is helping Caliban. Caliban plotting for his own freedom makes him, in Prospero's eyes, an ungrateful devil.

As previously Prospero has taught Caliban his langauge in also some manners as he thought Caliban uncivilised according to his standards ,so now he regrets it because it seems Caliban has not learned anything at all..