Arthur Schopenhauer said this quote

Boundless compassion for all living beings is the surest and most certain guarantee of pure moral conduct, and needs no casuistry. Whoever is filled with it will assuredly injure no one, do harm to no one, encroach on no man's rights; he will rather have regard for every one, forgive every one, help every one as far as he can, and all his actions will bear the stamp of justice and loving-kindness.