A valid argument, frequently known as "the slippery slope", which is a form of reasoning based on the use of moral principles:
—The moral exclusion of Action X depends on Principle Y.
—The moral exclusion of Action Z depends on Principle Y.
—If we accept Action X as morally permissible, then we eliminate Y as a binding principle.
—If Y is not a binding principle, then we must logically accept Z.
—Therefore the admission of X as permissible logically entails the permission of Z.
If the form of argument given above is invalid, it must be because there are no such things as moral principles.