“Those that wish to be clean, clean they will be; and those that wish to be foul, foul they will be.”
The fairy-tale warning connects desired moral change with the direction a person repeatedly chooses, while the surrounding story makes that lesson part of Tom’s larger transformation.
Source details
The Water-Babies, represented by Project Gutenberg #1018. Tom recalls the mysterious Irishwoman’s warning while half asleep. The cleanliness language participates in Kingsley’s Victorian Christian moral fantasy and is not a scientific claim about character or hygiene.