David Hume argues that in every system of morality, the author establishes the being of a God or makes observations concerning human affairs, but suddenly shifts to using 'ought' and 'ought not' propositions. This change is of the last consequence, as it expresses a new relation or affirmation that must be observed and explained. Hume recommends that authors use this precaution, as it would subvert all the vulgar systems of morality and show that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.