You must, therefore you can. A free will and a will subject to moral laws are one and the same thing. - Immanuel Kant

What would proceed from a continual promotion of living force, which does not let itself climb above a certain grade, other than a rapid death from delight?

If man is not to stifle his human feelings, he must practice kindness towards animals, for he who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.

Handle nur nach derjenigen Maxime, durch die du zugleich wollen kannst, dass sie ein allgemeines Gesetz werde.