  In the Treatise he argues against the epistemic thesis (that we discover good and evil by reasoning) by showing that neither demonstrative nor probable/causal reasoning has vice and virtue as its proper objects. Demonstrative reasoning discovers relations of ideas, and vice and virtue are not identical with any of the four philosophical relations (resemblance, contrariety, degrees in quality, or proportions in quantity and number) whose presence can be demonstrated. Nor could they be identical with any other abstract relation; for such relations can also obtain between items such as trees that are incapable of moral good or evil. Furthermore, were moral vice and virtue discerned by demonstrative reasoning, such reasoning would reveal their inherent power to produce motives in all who discern them; but no causal connections can be discovered a priori.  Causal reasoning, by contrast, does infer matters of fact pertaining to actions, in particular their causes and effects; but the vice of an action (its wickedness) is not found in its causes or effects, but is only apparent when we consult the sentiments of the observer. Therefore moral good and evil are not discovered by reason alone.