There are few figures in the history of philosophy as philosophically forceful as David Hume (this can be seen as a strength or a weakness, depending on your perspective). In his writings, Hume is a no-nonsense philosopher: he makes a great effort to follow his ideas to their logical conclusions without invoking God or anything else. Hume is unique in his writing in that an unquestionable search for why things are the way they are comes through. He really wanted to know what makes us tick, regardless of personal feelings or inherited beliefs. Hume was also known to be an incredibly friendly and sociable person, which isn’t normally the case with philosophers. I had a professor once who said, if he could meet anyone in history and have a conversation with him or her, it would be David Hume.

Hume wanted to take philosophy further away from metaphysics and closer to epistemology; he wanted to examine reason itself with more detail and consistency than other enlightenment philosophers (John Locke or George Berkeley). Hume is often seen as a skeptic, insofar as he saw problems with some our most cherished beliefs and the accepted systems of previous philosophers.

Impressions and Ideas

Hume held that the most significant thought is still duller than the most insignificant sensation. Thus he created the categories of impressions and ideas. An impression is an immediate perception. You are having an impression of your computer screen right now as you read these words. Now, turn your head and remember the computer screen you were just looking at. Now you’re having an idea of the computer screen, a less-lively perception. Every idea can be traced to an impression. What about an idea of something that doesn’t exist like a Unicorn? A unicorn is what Hume called a complex idea: it is made up of our idea of horse and our idea of horn. Thus, it is a white horse with a horn. While a Unicorn itself cannot be traced to an impression, the ideas that make it up can.

There is No Self

Hume came up with an interesting argument that there is no self. Since all ideas supposedly can be traced to an impression, Hume searched for an impression of “self” and could find no solid entity (see Soccio p306-307). At least not in the way an impression of “red” leads me to an instance of that color in the physical world. Try it now. Where is your self? When you reflect upon what you consider to be yourself, what do you find? Is there any unchanging entity? For Hume, there are only fleeting impressions and ideas—nothing that is evidence of some unchanging self.

As Soccio points out, Hume’s point here does echo Buddhism (in fact, a fellow grad student of mine did some research comparing Hume’s idea of the self to certain Buddhist principles). Buddhists believe that there are two levels to reality: this is known as the “two-fold truth.” At the superficial level are ordinary things: chairs, tables, people. But at the true level of reality (or “emptiness”) is a world of changing, interconnected entities. So at the deeper level of reality there is no self since things are always changing, constantly in flux.

In philosophy there is an area of research called “Personal Identity.” It deals with questions about the self and what makes us the same person from one day to the next. Our bodies are constantly losing cells and we are constantly changing and revising our beliefs. How is it that I am the same person I was five years ago, or even a week ago? (Later in the semester we will look at Personal Identity in relation to video games. Sound wierd? Guess you'll just have to wait...)

Hume’s Limits

As much support as Hume put in our faculties of reason, we must remember that he was also a skeptic, and he knew that our perceptions and methods of dealing with the world are limited in many ways.

Hume thought (as George Berkeley did) that there is no proof of an external world; all we have are impressions and ideas. Yes this is like it sounds: there is no proof that the world around you exists at all, it could all be an illusion as Descartes pointed out. You're having impressions of the computer in front of you, but the computer and all these words you are reading could simply be false representations of your senses. What else do you have but your senses?

The question here becomes, why do people believe in an external world? Why do the majority of you think that you are in fact looking at a real computer and that it exists? Why has the notion of an external world persisted for so long, and why does it continue to persist? Because the mind creates patterns to make the world coherent to us. No one has visited every city and town on the earth, but we still believe the earth to be a whole, we still have some sort of notion of a full, round earth in our heads. This is because you’ve seen a town here, a city there, a satellite image there and your mind has created a coherent whole out of these patterns. So says Hume.

Cause and Effect

In logic, reasoning is typically divided into inductive and deductive. Inductive reasoning is often a large part of scientific theories. It is reasoning from some particular idea or event to a general idea or event. Inductive reasoning is like a prediction. You are essentially saying that because some event in the past was a certain way, some other similar event in the future will be a certain way. When you see a swan that is white, then you predict that the next swan you see will be white, you are reasoning inductively. Clearly, there are differing levels of probable truth for this sort of reasoning.

Hume pointed out a problem with inductive reasoning, with assuming that one thing will follow another simply because it has followed it in the past. Up to his time, scientists thought there was a necessary connection between cause and effect. Hume points out that there is no necessary connection at all. We only think something is the cause of something else because it’s been the cause of something else in the past. It’s true that in the last five years, every time I’ve stepped on the pavement in front of my apartment, it has been solid underneath my feet. But will this necessarily always be the case? Isn’t it possible that there is an earthquake, or maybe aliens crash land on the pavement (ok, so the second is less likely)? Hume's point is that it's only out of habit that we assume one thing causes another. Really there is nothing necessary going on: there is one event in time followed by another event in time. Because that one event has followed the other so often, we think it will follow again out of habit. But it might not.

Hume's Take on the Argument from Design

Do you remember the argument from design when we discussed Aquinas? The argument from design says, basically, that we observe complex machines on earth (like watches) and they have a designer. The world itself appears as a complex machine, therefore it must have a designer and that designer is God. Hume’s problem with this argument is that it appears to be a false analogy. That is, he doesn’t like that the argument compares universe-creation with human-creation. (To create your own universe, see the instructions to the left. Good luck.)

Moreover, the things humans create are often imperfect. Imagine all the terrible designs of houses, bridges, and other feats of human construction that have fallen due to being poorly built. If our universe is anything like human-created things, how are we to know whether or not this universe is some failed model that God gave up on? Furthermore, Hume points out, the argument from design doesn’t specify what sort of God exists. Is it the Judeo-Christian God? Zeus?

Do you see what Hume is doing here? As I noted, he's following an idea to its farthest possible logical conclusion. He's asking, what are the implications of saying that the universe must have been designed? And who can we compare universe-design to but human-design? And if we make this comparison, how strong can it really be?

Reason is the Slave of the Passions

Following upon Hume’s insistency on the limits of reason is his notion that reason is the slave of the passions. He believed that reason is important; he just believed that reason is subordinate to emotions. Think about this for a second. How often do you get so worked up in some emotional state that you completely ignore reason? This happens when we are angry or nervous all the time. Alternately, how often do you use reason solely because you know it will grant you some future higher emotional state? Ever gone out on a date? In fact, recent research in brain science has uncovered the fact that we are much more emotional than we think we are. A good book that discusses some of this research for the layman is How We Decide by Jonah Lehrer.

Our moral sentiments, too, go beyond reason for Hume. Have you ever felt like something is “just wrong”? This is the sentiment Hume is talking about. Do you ever try to reason about it, or do you just have that feeling of wrongness? For Hume, morality corresponds to what we find agreeable and disagreeable. Why do we think it’s wrong when people get hurt? Because it causes pain.