Full Idea
Cruel treatment of animals is intimately opposed to man's duty to himself; ...for it dulls his shared feeling of their pain and so weakens and gradually uproots a natural predisposition that is very serviceable to morality in relations with other men.
Gist of Idea
Cruelty to animals is bad because it dulls our empathy for pain in humans
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 443 I.I)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.238
A Reaction
This idea is quite shocking. Kant's rough contemporary Bentham was far more enlightened. If we could be certain that our feelings of empathy for pain were not dulled by cruelty to animals, then it would be fine.