Hume on Causality and Justification

Introduction

In our last lesson, we confronted "Hume's problem of induction," a profound skeptical challenge to the foundations of our knowledge. We saw that our expectation for the future to resemble the past—a principle underlying all science and daily life—cannot be justified by reason. Instead, Hume concluded it's a product of non-rational custom or habit.

Today, we drill down into the very heart of that argument: the idea of causation. When you say a lightning strike causes thunder, you are making an inductive inference. Hume argues this is the only kind of reasoning that can take us beyond our immediate senses and memory. Therefore, to fully grasp his skepticism, we must dissect his analysis of causality itself.

This lesson directly targets the learning outcome: Evaluate Hume's argument against the rational justification of causal inference. This involves not just understanding his argument, but critically assessing its structure, its psychological claims, and its radical implications for how we understand the world.

1. Deconstructing the Idea of a Cause

Think back to the billiard balls. We see one ball roll, it makes contact with another, and the second ball rolls away. We intuitively say the first ball caused the second to move. But what is actually contained in this idea of "cause"?

Hume identifies three components to any causal relation:

1. Contiguity: The cause and effect are close to each other in space and time. The first ball must touch the second.
2. Temporal Priority: The cause happens before the effect. The first ball moves before the second one does.
3. Necessary Connection: This is the crucial, and most problematic, element. We assume there is some "power," "force," or "necessity" in the cause that makes the effect happen. It's not just that the effect does follow the cause; we feel it must follow.

The first two components are easily observable. We can see two events are next to each other and that one happens first. But what about necessary connection? Do we ever actually see the "power" of the first ball being transmitted to the second? Hume's answer is a firm no.

This leads to the central question of this lesson: If we never observe a necessary connection, where does our idea of it come from?

2. The Negative Argument: We Do Not Perceive Necessity

Hume's first step is to systematically search for the impression from which our idea of necessary connection could be copied. If he can't find one, then according to his own empiricist principles, the idea might be meaningless.

He looks in two places: the external world and the internal world.

a) The External World:
As we've noted, when we observe two objects interacting, we only perceive their conjunction, not their connection. We see one event follow another, but we never perceive the underlying power that links them.

The following video explains this part of Hume's argument very clearly.

b) The Internal World:
Some philosophers might argue: "I can't see power in the external world, but I can feel it in myself. When I decide to raise my arm, I experience the power of my will causing my body to move."

Hume dismisses this as well. He argues that we don't experience the force of our will directly. We are only aware of one event (the feeling of "willing") being constantly conjoined with another event (the arm's movement). We have no direct impression of the mechanism or power that connects the mental act to the physical one. People who are paralyzed, for instance, can have the first event (the willing) without the second (the movement), proving there is no self-evident, necessary connection between them.

The conclusion of this negative phase of the argument is that we can find no impression of "necessary connection" either in the world or in our own minds.

3. The Positive Argument: The Psychological Origin of Necessity

So, if the idea of necessity doesn't come from observing a power in the world, where does it originate? Here, Hume offers his radical, psychological solution.

1. We observe a constant conjunction of two types of events, A and B. For example, every time we've seen fire (A), we've felt heat (B).
2. This repetition creates a custom or habit in our minds. Our imagination forms a strong association between the idea of A and the idea of B.
3. Because of this habit, when we next perceive an instance of A, the mind is automatically carried to the idea of B. Hume calls this a "determination of the mind".
4. This feeling—this internal, psychological transition from one idea to another—is itself a new internal impression.
5. This internal impression of expectation is the origin of our idea of necessary connection.

In short, the "necessity" we attribute to causal laws is not a feature of the external world. It is a projection of our own mental habit, a feeling of expectation that we project onto the objects we are observing.

This video provides an excellent summary of this crucial move in Hume's argument.

4. Evaluating Hume's Argument and Its Implications

Now that we have reconstructed Hume's argument, we can begin to evaluate it. The conclusion is that our belief in causal connections is not based on reason but on a psychological instinct. What are the consequences of this?

Implication 1: A New Definition for "Cause"

If Hume is right, then our old definition of a cause as something containing a "power" to produce an effect is baseless. He offers two new, empiricist-friendly definitions:

• Philosophical Definition: A cause is "an object, followed by another, and where all the objects similar to the first are followed by objects similar to the second." This definition only refers to observable regularities (constant conjunction).
• Natural Definition: A cause is "an object, followed by another, and whose appearance always conveys the thought to that other." This definition includes the psychological component—the habit of mind.

Notice that neither definition refers to any mysterious, unobservable "powers." He has redefined causation in terms of what we can actually experience.

Implication 2: The Status of Scientific Laws

What does this mean for science? If Hume is correct, then a scientific law like Newton's law of gravity is not a description of a necessary force that pulls objects together. It is an extremely precise and reliable summary of a constant conjunction we have observed in the past. It tells us that unsupported objects fall, but it can never tell us why they must.

This view implies that scientific truth is provisional, always subject to revision based on new experiences, a point made in the following video clip.

Critical Assessment

Now, let's evaluate. Is Hume's argument convincing?

• Is it plausible? Hume's account of how we form causal beliefs through habit and expectation is psychologically powerful. It explains why we (and animals) are so good at learning from experience without engaging in complex logical proofs.
• Is it satisfying? Many philosophers have found Hume's conclusion unsatisfying. When you search for the cause of a software bug, you are looking for more than just a regular correlation; you are looking for the specific line of code that makes the program fail. You might feel that Hume has explained away the very thing we mean by "causation" rather than accounting for it. He shows that we can't rationally justify our belief in necessity, but does that mean necessity doesn't exist?

Hume's argument rests on his strict empiricist principle: every idea must be copied from a prior impression. If you accept that premise, his conclusion is very difficult to escape. If you deny it, you need to explain how we can have knowledge of things (like causal power) that we never directly experience. This is a central tension that will animate philosophy long after Hume, particularly in the work of Immanuel Kant.

Conclusion

In this lesson, we have moved from the general problem of induction to Hume's specific and devastating analysis of causal inference.

Key Takeaways:

• Hume argues that we never perceive necessary connection, the supposed "power" that links a cause to its effect.
• The idea of necessary connection does not come from an impression of the external world, but from an internal impression: the feeling of expectation or "determination of the mind" created by custom.
• Causation, therefore, is not a feature of objects but a projection of our own minds. Our belief in it is psychologically determined, not rationally justified.
• This redefines scientific laws as summaries of observed regularities, not descriptions of nature's hidden powers.

Preview of the Next Lesson:

Hume has now used his empiricist method to challenge our rational grounds for believing in the uniformity of nature and the existence of causal powers. In the next lesson, he turns this skeptical lens inward, asking a question that is perhaps even more fundamental: What is the "self"? Do we have any rational justification for believing we are a single, enduring identity that persists through time? We will examine his famous "bundle theory" of personal identity.