Causation: constant conjunction

This lesson turns Hume’s microscope on the single most important idea in his philosophy: causation. When we say the flame caused the heat, we mean far more than “flame, then heat.” We mean the flame had a power that necessarily produced the heat — a real, binding connection. Hume grants that this idea of necessary connection is the very core of causation. His question, as always, is his test of meaning: from what impression is this idea derived?

So he examines a single instance of causation as closely as he can — say, one billiard ball striking another. What can he actually observe? He finds only:

• Contiguity: the two objects touch, or are near, in space.
• Temporal priority: the cause comes before the effect.

He looks hard for a third thing — the power, the force, the necessity binding cause to effect — and he cannot find it. No impression of a connection ever appears. In a single case, Hume says, all events seem “entirely loose and separate”: one thing follows another, but we never perceive any tie between them (Enquiry VII).

But experience adds one thing over time. Watching many collisions, we notice that this kind of event has always been followed by that kind — a relation Hume calls constant conjunction. Crucially, repetition adds nothing new to the objects themselves: the hundredth collision looks exactly like the first. No hidden power becomes visible on the hundredth try that was absent on the first. So the extra ingredient repetition supplies cannot be found out there in the world.

So where does the idea of necessary connection come from, if not from the objects? Hume’s answer is his masterstroke. After repeated conjunction, the mind forms a habit: on seeing the cause, it is carried — determined by custom — to expect the effect. That felt determination of the mind, the inner push of expectation, is a genuine impression — but an internal one, a feeling in us, not a perception of anything in the objects.

Then comes the decisive twist: the mind projects this internal feeling outward, spreading itself onto the world, and mistakes its own habitual expectation for a power residing in the objects. The “necessity” we think we see in the flame is really the felt necessity of our own inference, misread as a feature of nature. As Hume puts it, necessity “is something that exists in the mind, not in objects.” The connection is real — but it lives in the observer.

This diagnosis lets Hume give not one but two definitions of “cause” — because there are two things going on, an external pattern and an internal feeling:

• Definition 1 (the objects/constant conjunction): 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.” — Causation as pure regularity in the world.
• Definition 2 (the mind/inference): a cause is an object followed by another “and whose appearance always conveys the thought to that other.” — Causation as the mental determination to infer effect from cause.