Western philosophers and scientists traditionally believed that to know something fully one must know the cause upon which it necessarily depends. He notes that the causal relationship provides the basis for all reasonings concerning matters of fact; however, unlike the relations of ideas explored by mathematics, no judgments that concern matters of fact are necessarily true.
This is because we can always imagine, without contradiction, the contrary of every matter of fact (e.g., the sun will not rise tomorrow neither is nor implies a contradiction).
Complex impressions and ideas, such as the seeing or imagining of an apple, can be analyzed into their component parts.
Whereas all simple ideas are derived from and exactly represent simple impressions, many complex ideas are not, and so their veracity must be called into question.
When we describe an action, sentiment, or character as virtuous or vicious, it is because its view causes a pleasure or pain of a particular kind.
Hume is well aware that not all pleasures and pains (e.g., the pleasure of drinking good wine) lead to moral judgments.
Common to this tradition is the view that knowledge is founded upon sense-perception, which the human mind passively receives.
But whereas Locke and Berkeley believe that human knowledge can go beyond sense-experience, Hume contends in the Introduction of his Treatise that our knowledge is limited to sense-experience, and so offers an empiricism that he argues is more consistent than those of his British predecessors.
Second, reason can deliberate about means to an end that we already desire.
But should reason be in error in either of these areas (e.g., by mistaking an unpleasant object for one which is pleasant, or by mistakenly selecting the wrong means to a desired end), it is not a moral but an intellectual failing.