Causation: What Are Causes?

Author: Dan Peterson
Category: Philosophy of Science, Metaphysics
Wordcount: 998

Suppose every time you drank almond milk, you got sick. The first time you got sick, you might not know why. But eventually, you might come to think almond milk was causing your sickness: maybe you’re allergic to almonds.

But what is a cause anyway? What is it for something to cause something else? Answers to these questions matter for scientific explanation and prediction as well as holding others responsible for what they do.

This essay presents three influential philosophical theories of causation.

1. Constant Conjunction and Theories of Causation

To understand theories of causation, it helps to understand the concept of a constant conjunction. Conjunctions are series of events—things that happen—where one event happens after the other. A constant conjunction is when one event always follows —is conjoined with—another event, as far as anyone has observed.

Constant conjunctions matter because A causing B  seems to require that B always follows A. For instance, if you get sick only once after drinking almond milk, then drinking almond milk doesn’t cause your illness.^[1] But causation must be something more than just constant conjunction since some constant conjunctions are just accidents: maybe you always drink almond milk when you eat dates, and dates make you ill.

Most philosophers, following Scottish philosopher David Hume (1711-1776), understand causation as constant conjunction plus something else.^[2] Theories proposing that this extra condition is something mind-dependent are called Humean—since, as we will see, Hume held such a view; theories referring to facts about the world outside of our minds for this extra condition are called non-Humean.

2. Hume on Causation

Hume believed that the extra condition is something our minds impose onto our observations. When we watch one billiard ball slam into another, we expect the second ball to move. Because we have seen many balls move after collisions with other balls, we anticipate this ball will move after another ball collides with it.^[3]

Hume was an empiricist about knowledge of the physical world: knowledge ultimately comes from some sensory experience.^[4] But we can’t see, or otherwise observe, a causal connection between the two balls;  we can only observe one ball rolling and then the next. So when we think that one ball’s motion causes the other’s, this is something our mind imposes on our observations: it is not an observation itself.

One challenge for Hume’s theory is this: we often try to determine what causes what—e.g., what causes various diseases. But if causation is just about our own psychological habits—and not about the world outside of our own minds—it’s unclear why we would, or should, care so much about causation. Those who follow Hume in their thinking about causation owe us an explanation here.

3. Causation and Laws of Nature

One non-Humean alternative to Hume’s theory relies on laws of nature to define and explain causation.^[5] Laws of nature are features of our world that connect facts in one region of space and time to those elsewhere.^[6] Frequently, laws of nature establish a necessary connection between a past and future state of the world: they tell us what the future must be given a specified past.^[7]

One simple non-Humean view defines causation as a constant conjunction where the (past) cause is necessarily connected to the (future) effect by a law of nature. Laws of nature ensure that effects must follow whenever their causes are present.

To return to the almond milk example, we might identify a law like “When someone allergic to almonds drinks almond milk, they are likely to be sick afterwards.” Then, the fact that you are allergic to almonds and the fact that you just drank almond milk would both constitute causes of your illness.

This theory seems promising, but it faces a problem: many conditions must be satisfied in order for laws to necessarily connect the past and future. For instance, the previously-mentioned law only connects your almond milk-drinking with your illness if other assumptions hold, like that you didn’t take medicine that would have neutralized your body’s response to the almond milk. Yet only your drinking almond milk seems to cause your illness; your not taking medicine doesn’t. We need some way to separate causes from non-causal background conditions like not taking medicine. Natural law-focused non-Humean theories of causation don’t clearly give us a way to do this.

4. Counterfactual Causation

Another theory of causation appeals to counterfactuals to anchor our understanding of causation.^[8] Counterfactuals are claims about what the world would be like if something were different. For instance, “If I were two feet taller, then I could dunk a basketball.”^[9]

This theory may be Humean or Non-Humean, depending on whether one thinks counterfactual claims are about how we judge the world to be or about how the world really is.

The Counterfactual Theory of Causation says that A causes B just in case there is a constant conjunction between them, and, if A did not happen, B would not happen.

On the counterfactual theory of causation, we’d say that the almond milk caused your illness when we additionally know that, if you had not consumed that almond milk, then you would not have gotten sick.^[10]

This theory aligns with some of the ways people think and talk about causation, but it also faces serious challenges. Here’s one: imagine everything in your refrigerator, including your almond milk, contains the same illness-inducing bacteria. So even if you didn’t drink almond milk, you would have gotten sick anyway from consuming something else illness-inducing. Counterfactual theorists cannot say that the almond milk caused your sickness because, if you had consumed anything else available, then you would still have gotten sick.

This is an example of preemption, where one cause blocks another cause of the same effect. Cases of preemption reveal that  counterfactual theories of causation require further refinement.^[11]

5. Conclusion

Though each theory has its advocates, all face challenges. Engaging these challenges and better understanding causation is essential given how common, and important, it is to try to identify causes in daily life, the sciences, and more.

Notes

^[1] Hume is called a “regularity theorist,” which means he believes that the relationship between cause and effect is a regularity, a pattern that always repeats in nature the same way. So, for Hume, effects always follow their causes. But not all causal theories agree with Hume about this —see, for instance, Hitchcock (2021) for a discussion of probabilistic causation, which happens when effects often, but not always, follow from their causes. (For an introduction to different understandings of probability, which could be applied to theories of causation, see Interpretations of Probability by Thomas Metcalf). You may find such theories appealing if you disagree with the claim that almond milk doesn’t cause your illness if you only get sick sometimes after drinking it.

^[2] See Hume’s A Treatise of Human Nature, specifically 1.3.6-10, and Hume’s An Enquiry Concerning Human Understanding, specifically sections 4 and 5 of the first enquiry, for Hume’s discussion of causation.

^[3] Specifically, according to Hume, it is the psychological forces of custom and habit that lead me to expect this outcome. See section 5 of Hume (1748).

^[4] For a brief introduction to empiricism, see Epistemology, or Theory of Knowledge by Thomas Metcalf.

^[5] See Laws of Nature by Michael Zerella for more on what these might be. Mill (1843) is one of the first to prominently ground his understanding of causation in laws of nature. Mackie’s (1965) theory of causation offers a more nuanced update of Mill’s approach. I follow (and simplify) Mackie’s approach in this section.

^[6] Not all philosophers understand laws of nature as capturing facts about the world as opposed to facts about our minds; the “best systems” account of lawhood, for instance, holds that laws of nature differ from other kinds of claims we might make because of the central role laws of nature have in our thinking about the natural world, not because they have some independent, physical reality. Those who think about laws this way could use laws to understand causation too, but the resulting theory would be a Humean, not a non-Humean, theory of causation. See Carroll (2020) for more on this and alternative theories of laws of nature.

^[7] Not all laws of nature conform to this generalization—for instance, a law of nature might establish a necessary connection between two regions of space (for instance, a law might tell us how strong an electric field is one meter away from an electric charge of a certain strength), or it might establish only a probabilistic connection between some past state of the world and the future (for instance, a law might tell us a radioactive isotope has a certain percentage chance of decaying in the next year).

^[8] The most popular and thorough contemporary counterfactual theory of causation comes from Lewis (1973).

^[9] Counterfactual claims are common but tricky to assess because it’s not always apparent what makes them true or false. I am not over seven feet tall, for instance, so what makes it true that, if I were, I could dunk a basketball?

One way to understand what makes true counterfactual claims true is to think about possible worlds: see Possibility and Necessity: An Introduction to Modality by Andrew Leo Rusavuk for an introduction to this concept. Possible worlds are ways that the world could be; they are large, consistent sets of claims about all of the things in the world: so, there are possible worlds where I can dunk a basketball, possible worlds where I can’t (this is a special possible world called the actual world), and perhaps even a possible world where I am a basketball (unless this is impossible and so there is no such possible world!): see Origin Essentialism: What Could Have Been Different about You? by Chad Vance.

Some possible worlds are said to be “closer” to the actual world than others. Worlds closer to the actual world are ones that share more facts in common with the actual world than worlds that are farther away. For instance, the possible world where I can occasionally dunk a basketball is closer than the possible world where I am a professional basketball player who can nearly always dunk a basketball.

We can use this language of possible worlds to help us understand when counterfactuals are true or false. A counterfactual of the form “If A were true, then C would be true,” is true just in case, in the closest possible world where A is true, C is true. This counterfactual is false just in case, in the closest possible world where A is true, C is false. For more on counterfactuals, see Starr and Kocurek (2025)

^[10] It is worth noting that we can complete the circle by clarifying the relationship between laws of nature and counterfactuals—many philosophers take laws of nature to justify which counterfactual claims are true and which are false. There are even theories of natural laws that define these features in terms of counterfactual claims. For one example of such a theory, see Lange (2009).

^[11] Cases of preemption are related to a similar challenge for counterfactual theories of causation, overdetermination, which occurs when many different causes seem to lead to the same effect instead of one cause blocking another (as happens in the almond milk example). An example of overdetermination is a firing squad. Imagine a prisoner is executed by being shot by 100 different bullets fired from 100 different rifles. Per the counterfactual account of causation, it seems wrong to call any one of those bullets the cause of the prisoner’s death since, had that bullet not been fired, the prisoner would still have been shot by 99 bullets and would still have died. Yet this fact seems to lead to the conclusion that none of the bullets caused the prisoner’s death, which is also clearly wrong. For more on challenges to counterfactual theories of causation and some of the responses to them that counterfactual theorists have come up with, see Menzies and Beebee (2024).

References

Carroll, John W., “Laws of Nature,” The Stanford Encyclopedia of Philosophy (Winter 2025 Edition), Edward N. Zalta & Uri Nodelman (eds.)

Hitchcock, Christopher, “Probabilistic Causation.” Stanford Encyclopedia of Philosophy (Spring 2021 Edition), Edward N. Zalta (ed).

Hume, David (1739). A Treatise of Human Nature. Urbana, Illinois: Project Gutenberg. Retrieved November 7, 2026.

Hume, David (1748). An Enquiry Concerning Human Understanding. Urbana, Illinois: Project Gutenberg. Retrieved November 7, 2026.

Lange, Marc. (2009). Laws and lawmakers: Science, metaphysics, and the laws of nature. Oxford University Press.

Lewis, D. (1973). Causation. The journal of philosophy, 70(17), 556-567.

Mackie, J. L., 1965, “Causes and Conditions”, American Philosophical Quarterly, 2(4): 245–264.

Menzies, Peter and Helen Beebee. (2024) “Counterfactual Theories of Causation”, The Stanford Encyclopedia of Philosophy (Winter 2025 Edition), Edward N. Zalta & Uri Nodelman (eds.)

Mill, John Stuart, 1843, A System of Logic, Ratiocinative and Inductive: Being a Connected View of the Principles of Evidence, and the Methods of Scientific Investigation, two volumes, London: Parker.

Starr, Willow and Alex Kocurek (2025), “Counterfactuals.” The Stanford Encyclopedia of Philosophy (Winter 2025 Edition), Edward N. Zalta & Uri Nodelman (eds.).

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About the Author

Dan Peterson is a visiting assistant professor of philosophy at Morehouse College. He received his Ph.D. from the University of Michigan and specializes in the philosophy of physics, philosophy of science, and formal epistemology. He has research and teaching interests in metaphysics, philosophy of religion, philosophy of education, and ethics. He is also the co-founder of Mind Bubble, an educational nonprofit in Atlanta that provides local students with free tutoring and educational workshops. DanielJamesPeterson.com

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