A Treatise of Human Nature

David Hume starts his treatise by examining the very basic question of how humans perceive the world around them and form ideas. First, Hume argues that there are two different types of perceptions: impressions and ideas. Impressions are “all our sensations, passions and emotions, as they make their first appearance in the soul” (49). While impressions come from what we experience with our five senses and from our emotions, ideas come out of our thinking and our use of reason (49). Hume admits that in cases where someone has delirium or is dreaming, the distinction between ideas and impressions may become blurred. Still, Hume asserts ideas and impressions generally differ from each other (49-50). Also, Hume breaks down perceptions into two other sub-categories: simple and complex. Simple perceptions are ones that cannot be broken down further, like a color. An example of a complex perception is an apple, which combines several simple perceptions like the apple’s taste and smell (50).

Through the imagination, it is possible for a person to create ideas combined from perceptions that a person never experienced. He gives the example of New Jerusalem, an ideal city from his imagination, or Paris, which he has visited but cannot have a perfect idea of that reflects the reality. Nonetheless, Hume argues simple perceptions correspond with each other, like how our idea of red corresponds with the sight of the color (51). These simple perceptions can combine to form ideas, but ideas cannot create impressions. For example, we cannot get a child to form an accurate idea of the color orange or crimson by describing the color to them. Instead, we would show the color to them.

Hume also gives the example of the taste of a pineapple. It cannot be understood through an imagined idea, but can only be understood through experience of its taste. In sum, “our impressions are the causes of our ideas, not our ideas of our impressions” (53). Hume admits it is possible that a person who had never seen a certain shade of blue before could be shown a gradient scale of different shades of blue and form an idea of what a missing shade might look like based on that (53).

Hume defines two types of impressions: sensation and reflection. Examples of sensation include pleasure, hunger, or heat. A reflection or secondary impression includes desire, fear, or hope (55). While sensations come out of physical reactions brought on by a person’s five senses, reflections come out of our ideas. Rather than being a response to one of the five senses like a sensation, a reflection is an impression formed by an idea that comes out of the memory or the imagination. The imagination also allows a person to take and mix together simple and complex impressions and ideas to form fantastic beings like giants that do not exist in reality (57).

Without the conscious control of the person, a person’s memory takes simple impressions and associates them with each other, forming complex ideas. It does this in three ways. The first way is resemblance, which is when two ideas appear like each other. The second way is contiguity, which is when an impression happened at the same time or in the same place as another impression. Finally, the third way is cause and effect, when an impression seems to bring about another impression. Impressions and simple ideas are unified in the memory in the form of complex ideas (60).

Next, Hume divides complex ideas into three categories: relations, modes, and substances. Relations themselves can be described as being based on resemblance (two objects that can be compared because of similar characteristics), identity (two objects that appear to belong to the same category), space and time (two objects are related to each other by distance or by time), and quality and degrees (like when there are two different shades of the same color). Also, there is contrariety, when two ideas are the opposite of each other, like warmth and coldness or darkness and light. This is not the same as difference, which is just “a negation of relation” (62).

As for the substances around us, Hume argues that they are “nothing but a collection of simple ideas […] united by the imagination, and have a particular name assigned them” (63). For example, our complex idea of gold comes out of our impressions of its color and its feel of softness, although we can later acquire more impressions of gold like learning that it can be dissolved in acid (64). Then, Hume discusses modes, which are expressions or arrangements of substances. Hume gives the examples of the concept of beauty or the action of a dance. Since modes are more abstract, our perceptions of them cannot actually change without changing what the mode itself is (64). This brings Hume to considering what he terms abstract or general ideas, like the abstract idea of a man versus the idea of an individual man. Hume asserts that our abstract ideas are based on our ideas of one thing. For example, our idea of a specific line gives us an abstract idea of all lines (66). When we imagine a large number of something, we only have the vague idea of that number and do not try to imagine the characteristics of everything that is part of that group. Our imagination instead enables us to form an abstract idea of any number of things or beings.

Hume argues against the theory formed by some philosophers that abstract ideas come from some perfect form of intellect or, as Hume describes it, “a kind of magical faculty in the soul” (71). When we actually do distinguish between individual things in a group, Hume argues we resort to a distinction of reason. Hume gives the example of white and black marble. The distinction of reason helps us tell that both pieces of marble are united through relations, but they both also have different characteristics, specifically their different colors.