A Treatise of Human Nature

Hume discusses the nature of knowledge. Looking over the different types of relations between ideas, Hume argues there are only four types of relations that can provide us with knowledge. These are contrariety (when two things are opposites of each other, like cold and hot), resemblance (when one thing resembles another), degrees in quality (such as shades of a color), and proportions in quantity or number (the amount or percentage of a thing) (118). As a result, Hume says that algebra and arithmetic provide “a precise standard, by which we can judge of the equality and proportion of numbers” (119). This is in contrast to geometry, which is less reliable because it relies on the general appearances of objects and lengths (118-19). According to Hume, even in geometry ideas are just copies of impressions. As such, ideas are “weaker” than the impressions they are based on (120).

Reasoning is based on the relations of ideas. For example, we reason causation or cause and effect by relating two ideas through contiguity (when two things are close to each other). Cause and effect can also happen because of priority (when something happens before its apparent effect) and succession (when there is a series of seemingly related events) (123-24). However, Hume still questions why an effect must always have a cause. He argues we cannot always prove that a cause led to an effect. For example, Hume discusses the assassination of Julius Caesar. Our entire understanding of this or any other historical event depends on ideas gained from historical documents and not on direct impressions (130-31).

On this point, Hume discusses how unreliable memories can be. Two people can experience the same event, but one remembers it better than the other person. The two talk, and the person who remembers it better can describe all kinds of details, but the other person still does not remember it well. In the end, though, the person with the better memory can mention some small detail, and then finally, based on that, the other person will finally remember everything. Nonetheless, Hume argues that ideas from memory are stronger than ideas from the imagination (133). This is proven, Hume says, by the fact that a painter who wants to express an emotion will try to see the expression from the face of someone experiencing that emotion rather than just rely solely on their imagination. However, memories decay over time, while ideas from the imagination can become repeated so often they become as vivid as memories, which is what happens with people who keep repeating a lie (134). For Hume, this proves why even an apparent cause and effect cannot be completely reliable.

Hume concludes that only experience allows people to distinguish the existence of one thing from another object. For example, we learn that heat comes from a flame through the sensation of the heat (135). This is an example of what Hume calls “constant conjunction,” when one outcome frequently or always seems to follow an object or an event. With the repetition of events, we form an idea of the “necessary connexion” (136) between two objects, like fire and heat. However, Hume adds that this is not because of anything having to do with the objects themselves, only their relation to each other. Also, we cannot determine the relation based on just pure reason, only experience. There is no reason based just on our thinking why fire always produces heat. Instead, we only gain an idea of that relation through direct experience (139-40).

Hume reiterates that an idea is only a copy of an impression. A belief simply gives that idea more power. So, Hume defines belief as “A LIVELY IDEA RELATED TO OR ASSOCIATED WITH A PRESENT IMPRESSION” (emphasis Hume’s) (144). He argues that what makes an idea a belief is how it is formed. Hume admits that he struggles to explain how a belief is formed and why it is different from another type of idea, except that a belief is something “felt” by the mind (146). Hume also argues that we use relations to strengthen ideas into beliefs. He gives the example of the Catholic Church. In Catholic rituals, there are images and practices that resemble the ideas in the minds of believers. By practicing these rituals, believers’ ideas are strengthened (149). Also, an impression that later becomes a belief may have been reinforced by what Hume calls “custom,” meaning repeated past experiences (152). Hume gives the example of a man trying to cross a river. The man never had the experience of drowning, but he has an idea of sinking formed from past experiences and observations (153-54). In another example, Hume describes how religious people and their beliefs in miracles increase when they see a holy location like Mecca or the Red Sea (160).

Hume next turns to the topic of credulity, which is when a person is too easily convinced of something. People become convinced of something, even something unlikely, when the experiences and arguments shared by other people become reinforced by the ideas in a person’s mind (162). Hume uses the belief in the afterlife as an example. Even though the afterlife or a “future state” (163) is beyond human experience, people claim to believe in it. Hume concludes that few people must actually believe in it or such belief comes out of ideas about death and anxieties about it (163-64). Also, through a person’s education, religious ideas like belief in an afterlife are impressed on people. Due to one’s upbringing and education, ideas can become strongly imprinted on a person’s imagination.  

Hume further views belief as linked to our emotions or passions. For example, a coward or a person who has depression may believe anything that encourages their emotions (169). The link between passion and belief is seen in fiction. Although most fiction has things in it that do not reflect reality, Hume argues that the most successful poems and stories must have elements that people can understand through their own experiences. This is essential for making fiction something that people can relate their own ideas to through their imagination (170-71). For this reason, Hume says a strong imagination susceptible to strong, but false, ideas can lead to both a talent in making fiction and to “insanity” (172). Still, Hume suggests that feelings are different and less vivid when they emerge from fictional ideas purely of the imagination than when they come out of experience and belief. This is why something that might excite us in fiction might instead be upsetting in reality (173).

Following this, Hume talks about chance and probability. Hume begins by considering a die, which has four sides marked the same way while the last two sides have another mark on them. Even with the marking from the four sides being more likely to show up than the marking with the two sides, we still cannot predict which marking will turn up with a roll of the die (178-79). However, Hume argues that even outside matters of chance, any sense of cause and effect we have is based on past experience. As with the die, we cannot rely on anything happening in the future, even if it has happened in the past (184). Based on this argument, Hume lists three “species of probability” (192) that have been discussed by other philosophers. The first is when there is not enough evidence from past experience to predict an outcome. The second one is when something has a different outcome than what someone predicts because of unknown causes. Finally, there is analogy, which is when people predict an outcome based on past experiences that only somewhat look like the present situation (190-93).

Hume goes on to name five more probabilities that have not been taken up by philosophers. The first is that if there was a recent event, we are inclined to think an outcome is related to the event just based on how recent it was. Second, the less vivid our memory of an event is, the less likely we are to connect it to a cause. Hume gives the example of someone with alcoholism who sees his friend die from the same addiction. At first, the person with alcoholism is determined to change his life in order to avoid a similar fate. However, as the memory fades, the less compelling such an outcome becomes (194). Third, the longer a series of copies of impressions and ideas, the less we trust the conclusion, except in cases where a chain of historical works copy without any variation the same fact, like the fact that Julius Caesar was a living person (195-96). Fourth, our past experiences can be misinterpreted and give rise to prejudice, which are irrational assumptions about groups of people. Prejudice comes out of what Hume calls “general rules.” Based on our past experiences, general rules are how we handle current situations. The best way to avoid prejudice and other misunderstandings is to base our general rules not just on experience, but also on present circumstances and an awareness of how much our “judgments” are formed just by “habit and experience” (197).

In conclusion, Hume returns to the broader topic of necessary connections. This is when we have an idea that two objects are “necessarily” connected together (205). Hume argues against some previous philosophical proposals to why there are necessary connections. Ideas and their connections do not come from pure reason, anything about the objects themselves, a deity who is “the prime mover of the universe” (209), or from our free will. For Hume, there is only ever one source for ideas: experience. Past experience gives us an idea of a cause and an effect. However, Hume concludes that the connections between the cause and the effect come from the mind, not from anything in the outside world. In other words, experience first gives us an impression of the relation between a cause and an effect. Then our mind itself generates an expectation that the cause will lead to the effect (217-18). From this, Hume defines causes as objects that are related to each other, or as the relations that the mind draws between two objects (220).

Finally, Hume lays out eight rules that govern cause and effect. First rule: cause and effect “must be contiguous in space and time.” Second rule: cause must precede the effect. Third rule: there is a “union” between cause and effect. Fourth rule: one cause must produce the same effect. Fifth rule: if multiple objects produce the same effect, it must be because these objects have some shared characteristic. Sixth rule: if two causes that resemble each other do not produce the same effect, it must be because of a difference between the causes. Seventh rule: if an effect increases or decreases with the increase or decrease of the cause, it should be considered a “compounded effect.” Eighth rule: a cause may not generate an effect alone, but needs to be assisted by some principle (223-24).

Afterward, Hume considers whether or not animals can reason. He concludes that animals also learn from experience, even if they are incapable of abstract reasoning. Hume views this as proof for his understanding of how human knowledge is based on experience (226-29).