An Enquiry Concerning Human Understanding

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. Impressions must always be of something that actually exists, but an idea does not have to be of an actually existing thing. It can be a synthetic coupling of two or more impressions that only exist within the imagination. These impressions are infinitely malleable since the imagination is capable of an endless succession of ideas.

Hume says that human reason is concerned with two things: relations of ideas and matters of fact. When it comes to matters of fact, only experience teaches us about cause and effect. We readily perceive that certain objects and events are typically connected to other objects and events, yet this is not the kind of thing that can be inferred from the start. If you were presented with a billiards table for the very first time, you wouldn’t realize that one ball can send another to the other side of the table. Only experience provides this knowledge.