A moral action, a crime, such as ingratitude, is a complicated object.
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“and if any of these parts be compared to two added to three, it will contain as many units as that compound number. But when you draw thence a comparison to moral relations, I own that I am altogether at a loss to understand you. A moral action, a crime, such as ingratitude, is a complicated object. Does the morality consist in the relation of its parts to each other? How? After what manner? Specify the relation: be more particular and explicit in your propositions, and you will easily see their falsehood.” source