Causality is the process of one thing “causing” another. That seems pretty simple: you throw a ball, the ball hits the window, it causes the window to break? What’s weird or confusing about that? What problems does philosophy need to solve where causality is concerned?
But things get more complicated when you try to think more abstractly about causality. What does the word “cause” actually mean? Can it be defined analytically? Does it correspond to anything objective, out there in the world, or do we human beings simply interpret the things we see as cause-and-effect?
Most philosophers would agree that your actions caused the window to break, but it’s almost impossible to prove this. We just instinctively “know” that the ball caused the window to break – it’s common sense, right?
Philosophers and scientists, however, understand that “common sense” is often extremely misleading – after all, common sense for centuries told us that the sun revolved around the earth! So common sense is not good enough as a philosophical argument. We need evidence.
When it comes to causality, however, it’s extremely difficult to find any hard evidence of one thing causing another. All we have is the evidence of two things happening back-to-back. But where is the evidence of a causal link between them? Can you give any evidence for this link other than just appealing to common sense?
This is hard to understand at first, because we’re so used to treating causality as something obvious – OF COURSE your actions caused the window to break. But take a step back and think about how you could possibly prove it. Maybe the window randomly shattered on its own, a split second before the ball struck. That certainly doesn’t seem likely, but can you prove that it didn’t happen?
Everyday people usually take causality for granted, because causality tells us we already understand it. But philosophers have been struggling for centuries to figure out how this common-sense intuition could be supported through a rigorous proof. As we’ll see, this such a proof is extremely difficult to imagine, and may even be impossible.
Does that mean causality doesn’t exist? Probably not. Most philosophers don’t think so. But it does mean that there’s a strange gap between what we know, and what we can prove we know. It illustrates that “knowledge” is not as closely linked with “logic” as we might like to think.
II. Causality vs. Correlation
Our popular notion is that causality can be defined by correlation. That is, if you threw a thousand baseballs at a thousand glass windows, the vast majority of them would break. This, then, proves that the baseball caused the window to break. Right?
Actually, it’s more complicated. While correlation sometimes implies causality, in many cases it doesn’t, and even in the best cases it doesn’t completely prove causality.
Statisticians have found that violent crime is correlated with ice cream sales. When more ice cream gets sold, there’s more violent crime; when ice cream sales go down, there’s less violent crime.
This is a strong correlation, but it doesn’t imply causality. Do the statisticians really think that ice cream causes people to run around committing armed robberies? Or, conversely, is it that criminals like to go get an ice cream cone after they rob someone? Neither story seems very likely. So what’s really going on here?
The answer is that there is a confounding variable: something else is going on in the story which is the real cause of the correlation. In this case, it’s weather. When it’s cold, people don’t buy as much ice cream and they also don’t commit as many violent crimes. During the summer months, the opposite is the case. So just because you see correlation doesn’t mean that one thing is causing the other.
This explains why causality is so philosophically complicated. Can you ever really prove that one thing caused another? After all, the connection between weather and crime is also based purely on correlation — so maybe there’s another confounding variable! Or maybe these correlations are just caused by coincidence!
Statisticians have a set of procedures for “proving” causality based on randomized trials. The randomness, they argue, will scramble up any confounding variables and thus demonstrate causality. This approach is probably the best way to understand causality, but it is still not philosophically complete! No amount of randomization can ever completely disprove the possibility that there are coincidences or confounding variables at work — it’s just a matter of proving causality to the best of our ability and being satisfied with that. Some philosophers and scientists feel that this is enough, while others find it unsatisfying.
III. Quotes about Causality
“Belief in the causal nexus is superstition.” (Ludwig Wittgenstein, Tractatus Logico-Philisophicus)
As we’ll see in the next section, there have been some philosophers over time who have argued that causality does not exist because it cannot be proven. Ludwig Wittgenstein, early in his career, was one of them: he argued that events follow other events in sequence, but that any causal connection between them is an illusion — admittedly it’s an extremely difficult illusion to shake, but Wittgenstein argued that it was an illusion nonetheless. Later in life Wittgenstein changed his views on many other topics, but he continued to be skeptical about causality (though he expressed his skepticism a little less aggressively!)
“The belief in causality is metaphysical. It is nothing but a typical metaphysical hypostatization of a well-justified methodological rule — the scientist’s decision never to abandon his search for laws.” (Karl Popper)
Karl Popper was a highly influential philosopher of science and one of the major thinkers who crafted our modern understanding of the scientific method. Popper agreed that causality could never be proved scientifically, but in the end, he argued, this was not important: scientists had to assume that causality was real in order to continue their search for an orderly understanding of the universe. Science has made great progress over the last few centuries, which suggests that its assumptions (e.g. causality) are more productive than other possible assumptions — but still, we should never forget that they are assumptions.
IV. The History and Importance of Causality
Philosophers have been debating the nature of causality for centuries and in many corners of the world: in India around the first century AD, there was a spirited debate between Astkaryavadins and Satkaryavadins over whether causality could be random or open-ended; even earlier, Aristotle had developed a notion of causality that would, centuries later, strongly influence the development of both Christianity and Islam. Thomas Aquinas, a Christian philosopher who was influenced by Islamic thought, used the Aristotelean notion of causality to “prove” the existence of God – he argued that everything that happens must have a cause, and so there must be a “first cause” that accounts for everything that has happened in the history of the universe. Aquinas called it God, but admitted that he couldn’t prove it was the Christian notion of God – his arguments supported some kind of religious belief, but didn’t specifically support Christianity as opposed to other religions.
In modern philosophy, debates about causality usually focus on two major figures: David Hume and Immanuel Kant. Hume, a philosopher of the Scottish Enlightenment, made a compelling case effectively proving that logic would never fully support the existence of causality. (His argument was a little like section 2 of this article, though obviously a lot more sophisticated!)
Many people take Hume at face-value, and interpret him as really and truly arguing against the existence of causality. This is a pretty radical view, and in the standard interpretation Hume is presented as an eccentric, an extreme rational thinker who denied the existence of causality because he couldn’t find a logical justification for it. His argument is summarized as: logic cannot prove the existence of causality, so therefore we must reject the existence of causality!
But Hume might have been trolling. By all accounts, he was an exceptionally jovial and humorous soul, not the sort of person you would expect to be a philosophical radical or extremist. In addition, several of his other books suggest that he still believed in causality in spite of the fact that he couldn’t prove it. On this interpretation, Hume’s argument was more about logic than causality. That is, we would reinterpret his argument to say: logic cannot prove the existence of causality, and causality is clearly real, so therefore logic is not perfect!
Kant came along a few decades after Hume and was impressed by the argument against causality. He later said that Hume “awakened me from a dogmatic slumber,” essentially giving Hume the credit for Kant’s whole career! Kant theorized that causality was part of the structure of thought itself, not an objective attribute of events in the world. In other words, he argued that you could never see causality occurring (or at least you could be sure that you were seeing it – “common sense” might tell you that you were seeing causality, but you’d never be able to prove it logically), but that as soon as you reflected on the events and tried to make sense of them, you would inevitably reach for the idea of causality. Causality, then, was like a tool: you had to think with the idea of causality, but that didn’t mean that you had to believe it was an objective truth.
V. Philosophy in Popular Culture
“You see there is only one constant. One universal. It is the only real truth. Causality. Action, reaction. Cause and effect…Choice is an illusion created between those with power and those without.” (The Merovingian, The Matrix Reloaded).
The Merovingian is an ancient program in the Matrix, and his age is revealed by the simplicity of his philosophy. For him, everything boils down to the single “fact” of causality. As we’ve seen, though, causality is far from a single, simple fact — for many philosophers, it is not “the only real truth” but actually an illusion or a superstition, even though it has such broad “common sense” appeal!
What the Merovingian is really talking about is not so much causality as determinism, which is a whole complex family of ideas based on the concept that everything is completely determined by causes in the past (meaning there is no free will). Whether these causes “exist” is up for debate.
“I used to think correlation implied causation. Then I took a statistics class. Now I don’t.”
“Sounds like the class helped.”
This is a joke from the webcomic xkcd. The joke illustrates how attached human beings are to the idea of causality — strictly speaking, you can’t prove that the class helped the other person understand causality better. It’s possible that there was some confounding variable elsewhere in the picture, and that this was the real cause of the outcome; it’s even possible that the whole thing is just a coincidence!
Of course, by far the most plausible story is that the class actually helped. The point of the comic is that, as Hume and Kant said, causality is essential to the way human beings narrate our lives and make sense of the world, and we all generally believe in it even if we can’t completely prove it.