Let a man's insensibility be ever so great, he must often be touched with the images of Right and Wrong
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“The difference, which nature has placed between one man and another, is so wide, and this difference is still so much farther widened, by education, example, and habit, that, where the opposite extremes come at once under our apprehension, there is no scepticism so scrupulous, and scarce any assurance so determined, as absolutely to deny all distinction between them. Let a man's insensibility be ever so great, he must often be touched with the images of Right and Wrong; and let his prejudices be ever so obstinate, he must observe, that others are susceptible of like impressions. The only way, therefore, of converting an antagonist of this kind, is to leave him to himself.” source