In An Enquiry…, Hume’s first sentence brings up the topic of moral philosophy, defining it as the science of human nature. The reader should be aware that Hume does not mean science the way that it is used in the 21st century, as referring to empirically verifiable methods of testing and demonstration in biology, physics, or chemistry. Hume uses science in a way that reflects its Latin origin in the word scientia, which is generally translated as knowledge. Hume speaks about moral philosophy as the means of gaining knowledge about human nature.

Hume divides the various perceptions of which the human mind is capable into two categories, contrasting thoughts and ideas with what he calls impressions. In contrast to ideas, an impression is the actual sense experience of touch, taste, hearing, emotions, or any other means by which the human person has a sensory experience. An impression is an experience that occurs in the moment, distinct from an idea about something experienced at some point in the past.

Ideas are the first category of human perception against which Hume contrasts impressions. Ideas are mental constructs or realities that are held in the memory and the imagination. They are perceptions not of a present experience but of an experience that happened at some point in the past.