David Hume was a Scottish philosopher in the mid 1700s. His philosophy was a culmination of British empiricism. This was a tradition that began with Francis Bacon and continued through Thomas Hobbes, John Locke and then was critiqued rather effectively by George Berkeley.
Berkeley had said that it is a wrong idea to think that we can know for certain what our sense perceptions say to us. Hume took this critique very seriously. He realized that Berkeley was correct, that our sense perceptions are highly variable. Locke had kind of admitted that, but still carried on with the idea that really, because we have nothing but our impressions, that really has to be the basis of our knowledge.
Well, Hume said this is not good enough. There must be a more firm basis for the sciences, and by the mid 1700s, science was everything within academia and scholarship. Hume, like Locke before him and Descartes before Locke, wanted to create a firm foundation for the sciences so that we could know for certain that what science seems to be telling us in its explorations is actually true.
For philosophy, Hume said that it seemed obvious that philosophy has not resolved anything. We are still having the same arguments that we have been having since Plato.
As for science, Hume was rather disturbed by the looseness that he saw. In science, people were just making observations, and if their observations looked like they gave them an answer, they accepted that as an answer, and that was not good enough for Hume.
The Nature of Human Perception
Hume said that one thing that both science and philosophy were neglecting to look at was human nature, and by that he meant simply how do humans think? How do humans perceive? So, he looked at this human nature of how it is that we perceive things.
Being in the British empirical tradition, he basically accepted Locke’s fundamental view that there are objects in the world, and those objects create sense impressions within our minds. Hume used terms differently than Locke did, and so one of the difficulties in understanding Hume’s epistemology is seeing how he has changed terms.
Locke talked about simple ideas of sensation and reflection. There are objects in the world. They create simple ideas of sensation. We are aware of the simple ideas of sensation that objects produce in our sense perceptions, and we reflect on those. And those create ideas of reflection.
But Hume labels the former “impressions,”—objects create impressions in our mind. Our reflection on the sense impressions that are produced by objects in the world Hume calls “ideas.” It’s a bit of just relabeling things. It’s not a fundamentally different concept, but if you take that into account, Hume and Locke have similar epistemologies. In their epistemologies, they agree that everything in the mind can be traced to sense experience, and every idea in the mind is a representation of objects of sense impressions.
The big question, of course, is are the representations of objects in our ideas true to what is out there in the world?
Hume is much more pessimistic than Locke was about the powers of the Hume mind. Hobbes, before Locke, had said that the mind is entirely passive, is just like a rock on the edge of the ocean, buffeted by the waves lack of knowledge that we have free will and that we can create ideas. Hume goes backwards a bit more to Hobbes in being a materialist and adopting the model of the passive mind. Locke, of course, believes in an active mind, as had William of Ockham before them.
But Hume said that the creative power of the Hume mind amounts to no more than combining resizing and switching around ideas provided by impressions. Each sent impression is distinct, but it exists for only a brief moment, disconnected from any other impression.
That right there is an important thing to keep in mind about Hume. Every impression, Hume says, is distinct, and it exists for only a brief moment. Now it immediately becomes an idea in the sense that it is a memory, and we deal with memories because as soon as an impression happens, it’s gone. We can only deal with the memory of the impression that we had. And as we shall soon see, this belief of Hume has profound consequences.
A Physics of the Mind
Hume wanted to identify a physics or science of the mind, seeking to find the bond of union or gentle force that constituted a kind of mental gravity that brought together particular ideas into association. We associate certain ideas. Fire is hot, grass is green, cows are animals, and so on. These ideas introduce each other with a certain degree of regularity.
Well, why did they do that? The simple answer would seem to be, well, that’s because those are what we experience. But again, Hume is not happy with that answer, because he believes that impressions are distinct.
Ideas with similar features tend to be associated with other because that is the physics of the mind. There are three principles, he says, of the association of ideas, resemblance, contiguity, and cause and effect. These three associations of ideas are what happens within our minds, that ideas seem to just kind of gather together, connect with each other.
This is very passive. This is not something that we are consciously doing, he says. But these gentle forces of association operate without any conscious willing on our part. Hume is not interested in asking why our ideas become associated. Why do we believe the glass is green? He doesn’t care. It’s just that happens.
So, in Hume’s view of the world, there’s no reason behind anything. Things just happen. We’re passive. We have no free will. We have no real creative power. Ideas are beyond our control. Ideas just happen to us controlled by mental forces, like natural objects are controlled by forces of nature.
Some scholars have speculated that Hume received many of these ideas from his study of Buddhism. Buddhism did say that we have no self, thoughts are illusions, the idea of a contiguity of self is an illusion. Of course, Buddhism had a very religious answer to that by saying that we need to negate the self in order to find salvation, salvation being, well, the negation of self.
Hume didn’t go that far. Hume wasn’t interested in religion, but he did take this idea that nothing is permanent; everything is transitory, at least in the Hume mind.
The Assumptions of Science
So, what does any of this have to do with science? Well, for Hume, looking at science and science’s effort to build structures of knowledge, Hume did not see this as possible. Hume said that science assumes so much.
There are three main assumptions of science, Hume says.
One is that the present and future will behave like the past.
The second is that we have impressions of causation that we see in the world cause and effect.
And the third is that we can reason from effects that we perceive to the causes that produce those effects.
These are three very big ideas, and Hume is correct in saying that science depends on these assumptions.
Again, present and future will behave like the past, so there’s a contiguity of sameness and physical laws throughout time.
The second is we have impressions of causations, that we see causation, we understand causation, and there’s a reason for us to believe that causation happens to objects in the world.
And the third is that because we have these first two assumptions, we can reason from the effects that we see to laws and realities that preceded what we perceive.
Okay, on the first assumption that the present and future will behave like the past. Our default, unthinking way of dealing with the world, Hume says, is to assume that if the impression of the table in a room today is identical to the impression of the table the last time we saw it in the room, then it is the same table. Well, we assume that we do, whether we’re scientists or not.
Hume points out, though, that we have no solid rational reason for our assumption that similarity of impressions equals similarity of identity. Just because that table looks like the table that you remember seeing before is not proof that it’s the same table. Someone could have taken away the previous table, switched it with a different table, and they may look the same, but they are not the same table.
Now this may seem a silly argument, but Hume is correct. Science is based on such an assumption, and Hume points out that it has no rational basis.
Now let me stop here and point out this issue of rational basis, which is something that Hume comes back to again and again. All of philosophy before Hume and up to Hume’s time believe very strongly that logic proves things, that Hume reason reaches knowledge, and that every idea that we have has a rational basis.
Science was operating under that assumption in Hume’s time and arguably much of science and much of human society seems to also operates under that assumption. My logic, my rationality, proves things. Everything has a rational basis.
Hume was saying that the problem for science is that all scientific beliefs in laws assume that the objects and forces operated the same in the past as they do now, and will continue to operate the same in the future.
But what rational basis is there for that? What logic proves that?
Hume was aware that William of Ockham had said for centuries earlier that logic proves nothing about the world. We have to observe the world, and our observations are just, Hume says. Impressions. They’re not proof of anything other than I’m looking at what looks like a table right now.
What if it is not the case that the present is the same as the past? Hume points out that if gravity started operating differently tomorrow, this would not violate logic.
This is difficult for people to handle, especially people who love science.
“Well, of course it’s going to, they say.”
Well, who says?
Hume’s arguments in the mid 1700s were directed at academia’s still lingering belief in the logical approach of medieval scholasticism. That approach said that if it makes sense logically, it necessarily is true.
If we’re honest with ourselves, though, rational argumentation is, beyond a few certain very simple syllogisms, more what we agree with, what we like. We agree with that argument, we like that argument, so we believe it and call it rational.
I, of course, know that this is difficult for a lot of people to accept what Hume is saying here, but work with me here. Listen to this a little bit longer because science’s second assumption is the target of Hume’s most devastating critique. That’s the assumption of causation.
Hume points out that we never have an impression of causation. In other words, we never see causation. Again, hear me out. This is a difficult concept, but this is important to what Hume’s trying to do here.
Hume’s argument is that the reason that you concluded, for example, that the hammer caused the pain in your thumb is because you had an impression of the hammer hitting your thumb, and immediately afterwards you felt an impression of pain in your thumb.
But as anyone who has had any education at all knows, correlation is not causation. And the impression of the hammer and the impression of the pain is correlation, not causation. We have not experienced the actual causation of the impression of the hammer on the impression of pain.
This can be seen and understood if we condition someone by shining a light before we secretly flipped a switch opening a door, the person not seeing us flip the switch would come to conclude that the light switching on is what caused the door to open. But this would be an incorrect assumption. The problem Hume points out is that everything we do is this type of incorrect assumption, and science depends on this incorrect assumption of causation.
A Game of Billiards
Here’s an example that Hume uses that I will describe more fully than he does. We’re going to consider a game of billiards.
In the game, we see a white ball moving in a straight line across the billiard table toward a red ball.
Then we see the white ball appear to come in contact with the red ball.
Then we see the red ball move.
All of this is happening. We’re seeing this. We experienced three separate independent impressions. Remember what Hume said? Every impression that we have is independent. It stands on its own.
But we assume that the white ball has caused the red ball to move well, why do we assume that, Hume asks. Where do we find the impression of causality? That the red ball moved is not the question. Why the red ball moved is the question.
It is an imagined assumption on our part that the white ball caused the red ball to move and it is no more, Hume says. Then the influence of custom to believe with certainty that it will do so again. It’s a kind of habit, a custom. We just assume. It’s a bit of lazy thinking on our part. We don’t bother to ask.
This is a difficult argument to grasp. I understand that. But work with me here.
Let’s look at another of Hume’s examples. Hume says that we have observed the sun rise every day in the past. All of our lives, the sun has risen in the morning, so we have come to expect that the sun will rise tomorrow.
But the relation between the fact the sunrise has always happened in the past and the proposition the sun will rise tomorrow is not a truly rational logical necessity. That is because the proposition, “the sun will not rise tomorrow,” is not a contradiction. It would not be contrary to reason if the sunrise did not happen tomorrow. Obviously, this would be a surprise to us, and it might be rather calamitous to us. But that is Hume’s eventual point. It’s by custom, by habit, we assume the future will behave like the past because we assume causation.
Back to the billiard balls. If we saw the white ball come in contact with the red ball, and the red ball did not move, would that be a logical contradiction? Well, can we imagine the red ball staying still when the moving white ball comes in contact with it? Of course we can. We can imagine that. And if we can imagine that, then it’s possible.
The only things that are impossible are things that are so logically contradictory that we cannot imagine them like a round square. We can’t imagine that, we can’t picture that, because it’s impossible.
But a red ball not moving when a white ball strikes it, we can easily imagine because it’s very possible. Hume says that we will find the same is true about every single cause and effect relation in which we believe.
“Now wait a minute, wait a minute.” you’re saying. “We can demonstrate that white balls striking red balls will cause the red ball to move.”
That’s a perfectly reasonable thing to say, isn’t it? Well, Hume would say you are making a mistake of reasoning.
You appeal to science, thinking, “Can’t we prove this scientifically?”
Well, OK, let’s construct a scientific experiment. This is what science does, right? We’ll conduct 100 trials of white balls striking red balls and record the results. So, we do this, and we record the data that 100 times the white ball struck the red ball and 100 times the red ball moved, and you feel, you feel this indicates your proposition that white balls striking red balls cause red balls to move.
All right, I say, what if we did a 101st trial? Would you say that the red ball will move again?
“Of course!” you reply.
Ah! But can you imagine the red ball not moving in this 101st trial? Yes, you can. It is not logically impossible for the red ball not to move when the white ball strikes it. Therefore, our experiment has not established a necessary causal connection.
A necessary causal connection would be that it is impossible for the red ball not to move. But it is possible for the red ball not to move, because all that our experiment has established is a record of past behavior. At most, it has recorded a pattern.
But we have never observed causation. We have never observed a cause-and-effect relationship. In each trial, we saw only three separate impressions.
We saw a white ball moving toward a red ball. That’s impression one.
We saw the white ball seem to come in contact with the red ball. That’s impression 2
And we saw the red ball move. That’s impression 3.
Three separate impressions, and that’s all we observed. We did not observe the necessary connection between these three impressions that establish causation.
Again, let’s go back to the example of shining a light and a door opening. If we had a subject in a room and we shine shone the light 100 times and the door opened 100 times, that person would have developed the habit, the custom of believing that the light is what causes the door to open. And yet, even though we fooled this person 100 times, it has never been the case that the white light caused the door to open. That lack of connection between impressions is what Hume is talking about.
Hume said that we imagine causation on the basis of our experiences. That doesn’t mean it’s stupid, but it is something we have created in our mind. We imagine causality because we experience contiguity between certain events. In this case, the white and red balls appear contiguous in space, and we experience a priority in time of impressions. We experience impressions in a certain order.
In this case, we always had experienced the order as 1, 2, and 3 as seen in the graphic here.
We never saw it in another order. It seemed to be a pattern. We also experienced them in constant conjunction, again, a pattern. We always witnessed impressions 1, 2, and 3 conjoined repeatedly in the same space in a sequence of time.
Now we did experience these things. Hume isn’t saying that this is an illusion. He rejected the ideology of Buddhism on that one. It’s not that the world is an illusion, it’s just that we can’t put the weight of rational necessity on our experience as science does.
We add, Hume says, to these impressions, the further idea that there is a necessary connection between these events. We observe X followed by Y. So, we infer that all similar occurrences of X will be followed by similar occurrences of Y. We infer it, and we’re not stupid for inferring it, but we cannot prove it as a certain relation of ideas. It is at most a probable correlation. But as any good scientist should know, again, correlation is not causation.
The Implications for Science and Us
Hume’s argument about causation has enormous implications for science. Everything in science is predicated on cause-and-effect relations, from the motions of the planets to the working of cells in the body. But if we never have an impression of causation, what faith can we place in our science?
Hume is challenging us. Is our faith in physics, chemistry, in biology and all the other sciences completely misplaced?
This is a big problem because as we see in Hume’s arguments, science not only assumes causation without a rational basis we can never prove it, but most of science’s arguments depend on assuming that from effects we can reason to causes. This is another big deal. This is science’s third assumption.
For example, a paleontologist finds fossil bones. He or she tries to determine what caused the bones. The bones we’re not disputing here. The bones are real. We have them. We’re seeing them. What caused them? Where are they from?
The paleontologist tries to answer these questions by looking at other bones, including from animals alive today, and tries to infer what animal in the past created the fossil bones. The paleontologist is doing science, and the paleontologist is relying on the three assumptions that Hume identifies causation. That we can know that a particular something caused these bones, that the past behaved like the present, that we can reason from present examples to what happened in the past, and that we can reason from an effect to a cause that we can infer from this bone, which is an effect, what caused the bone.
All other sciences make the same assumptions, and the assumptions are similarly intertwined. Within that reasoning from affect to cause is science’s assumption that the future will operate as it has in the past, And we’ve touched on this point previously, but it’s worth mentioning. There is absolutely no reason to believe, on the basis of either reason or experience, that even the most fundamental laws of physics will continue to operate in the future as they have in the past.
Now, Hume is not suggesting that we give up on science. He’s not antiscience. He’s a big believer in science, but he is suggesting that our science is not based on reason. This is important.
Science is not based on logic. William talked about that four centuries earlier and had been forgotten. Hume is reminding us that logic tells us nothing about the world.
Science, Hume says, is based on assumptions born of habit, not on reason. And this should concern us. We should care about this. Reason cannot prove our most fundamental beliefs about how the universe operates.
We must give up the pretension that our reason and science give us perfect knowledge. Instead, we must accept what Hume said. Knowledge is inference based on inexact ideas of fleeting impressions molded by social customs and personal habits.
So, what has Hume done? In seeking a science of the mind, he has instead found that reason tells us nothing about the world. We are nothing but a collection of customs and habits based on fleeting impressions. We have no basis for believing in anything.
But habit, he says, is what saves us. Literally.
It’s not exactly the answer Hume set out to find, but he says there’s no way around this. Rational philosophy has reached a dead end. British empiricism has reached a dead end. Science is nothing more than an artifact of human nature and social custom.
Doesn’t mean science is worthless. Doesn’t mean we stop doing science or philosophy. But we have to stop pretending that we’re doing something we’re not actually doing.