David Hume On Causality and Science

David Hume
David Hume

David Hume’s (1711-1776) philosophy was the culmination of the trajectory of British empiricism (see Chapters 8 and 11). Hume’s shocking conclusion was that the long quest for understanding had reached no true understanding at all. Descartes was not a true skeptic because he believed that knowledge is possible. Hume was a true skeptic in that he genuinely doubted that real knowledge was possible. Is scientific and philosophical knowledge possible?Yes and no, is what Hume’s philosophy tells us.

In Hume’s time, science was rebuilding the structures of knowledge, but he was disturbed by the looseness he saw in these structures. He was in no way against science, but he became aware that science makes assumptions that are on shaky ground. Like Berkeley’s argument against matter, Hume’s arguments against scientific reasoning have no easy refutation.

Hume says that there are three main assumptions of science: that the present and future behave like the past, that we have impressions of causation, and that we can reason from effect to cause. On the first assumption, our default, unthinking way of dealing with the world 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, that it is the same table. Hume points out that we have no solid rational reason for our assumption that similarity of impression equals similarity of identity. Someone could have switched tables that look the same. This may seem a silly argument, but science is based on such an assumption, and Hume points out that it has no rational basis. 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. What if this is not the case? Hume points out that if gravity started operating differently tomorrow, this would not violate logic. Much of Hume’s arguments in the mid-1700s is directed at academia’s still lingering belief in the logical approach of medieval Scholasticism.

Science’s second assumption is the target Hume’s most devastating critique: causation. We never have an impression of causation, Hume points out. Hume’s argument is that the reason that you concluded the hammer caused the pain in your thumb is because you had an impression of the hammer hitting your thumb and immediately afterward you felt an impression of pain in your thumb. But this 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 if we conditioned 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.

Example: A Game of Billiards

Here’s an example that Hume uses that I will describe more fully. 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. We experience three separate impressions. But we assume that the white ball has caused the red ball to move. Why? Hume asks where we find the impression of causality. That the red ball moved is not the question; why the red ball moved is. It is an imagined assumption on our part that the white ball caused the red ball to move, and it is no more than the influence of custom, Hume said, to believe with certainty that it will do so again.


This is a difficult argument to grasp, but work with me here. Let’s briefly return to Hume’s earlier sun example. Hume says we have observed the sun rise every day, so we have come to expect that the sun will rise tomorrow. But the relation between the fact “the sunrise has always happened” and the proposition “the sun will rise tomorrow” is not a necessarily true relation of ideas. 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 that would be a surprise to us, but that is Hume’s eventual point.

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, remember our litmus test. Can we image the red ball staying still when the moving white ball comes in contact with it? Of course; therefore, it is not logically necessary that the white ball will make the red ball move. We will find the same to be true about every single cause-and-effect relation in which we believe.

“Wait a minute,” you might say, “we can demonstrate that white balls striking red balls will cause the red ball to move.” Hume would say you are making a mistake of reasoning. You appeal to science, thinking, “Can’t we prove this scientifically?” Well, let’s construct a scientific experiment. We’ll conduct 100 trials of white balls striking red balls and record the results. 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 this vindicates your proposition that white balls striking red balls causes red balls to move. All right, I say, what if we did a 101st trial. Would you say the red ball will move again? “Of course!” you reply. Can you imagine the red ball not moving? 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. We have only recorded a pattern of past behavior. Nor have we ever observed causation, or a cause-and-effect relationship. In each trial, we saw only three separate impressions. We saw a white ball moving toward a red ball; we saw the white ball come in contact with the red ball; we saw the red ball move. That’s all we observed. We did not observe the necessary connection between these separate impressions that establishes causation.

Hume's billiard ballsHume said that we imagine causation on the basis of our experiences. We imagine causality because we experience contiguity between certain events; in this case, the white and red balls appear contiguous in space. We also experience priority in time. We experience impressions in a certain order; in this case, we always experience the order as 1, 2, and then 3 in our graphic, never another order. We also experience them in constant conjunction, meaning that we have witnessed the impressions 1, 2, and 3 conjoined repeatedly. These are real matters of fact, we have experienced them, and Hume says we add 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, 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, correlation is not causation.

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 workings of cells. But if we never have an impression of causation, what faith can we place in our science? Is our faith in physics, chemistry, biology, and all of the others misplaced?

This is a big problem because, as we can see in Hume’s arguments, science not only assumes causation without a rational basis but most of its arguments depend on assuming that from effects we can reason to causes. This is science’s third assumption. For example, when a paleontologist finds fossil bones, he or she tries to determine what caused the bones. The paleontologist looks 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 is relying on three assumptions: 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 [the effect] what caused the bone). All other sciences make the same assumptions and operate similarly. Intertwined within that reasoning from effect to cause is science’s assumption that the future will operate as it has in the past. We’ve touched on this point previously, but it is worth mentioning that 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.

Hume is not suggesting that we give up on science. He is suggesting that our science is based on assumptions born of habit, not reason, and that this should concern us. 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 that human knowledge is inference based on inexact ideas of fleeting impressions molded by social customs and personal habits.

Except from my book, The Quest for Understanding: A Historical Introduction to Philosophy, Kendall Hunt Publishing.

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