In David Hume’s An Inquiry Concerning Human Understanding, he includes a section on the connection between cause and effect. He draws examples such as one billiard ball moving and striking another, then the second ball moving. Hume goes to some length to convince us that we have absolutely no idea of why one event would cause another. All we have, he says, is a sequence of events that customarily follow each other over repeated experiences.
We only find, that the one does actually, in fact, follow the other. The impulse one billiard-ball is attended with motion in the second. This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects: Consequently, there is not, in any single, particular instance of cause and effect, any thing which can suggest the idea of power or necessary connexion. (sic)
Hume mentions many times in this section that we have no idea of how a cause can be connected to an effect. How does heat come about from a flame? How are our limbs moved by our will? When a string vibrates and we hear a sound, we cannot know why we hear a sound, but merely that one customarily follows the other. Indeed, “even in the most familiar events, the energy of the cause is as unintelligible as in the most unusual, and that we only learn by experience the frequent Conjunction of objects, without being ever able to comprehend anything like Connextion between them.”
Hume was very insightful to make this observation. However, he confuses how we learn the connection with whether or not we can investigate it and prove that the cause generates the effect. Yes, we typically learn cause and effect from repeated observation. However, it is not true that the most common effects are as much a mystery as the most unusual and mysterious ones. When we encounter an effect, it may take us a while to learn why the cause generates it. Indeed, in the case of the human mind and will, we may never fully understand all aspects of the cause. But it is not the case that all cause and effect relationships are this way.
In his billiard ball example, we can understand and explain the physics of objects and motion. Newton helped us demonstrate that objects with mass, when in motion, must expend that energy when striking another object. We know why the second ball moves because we know the laws of physics involved in objects in motion. Concluding that the second ball will move is not a case of post hoc ergo propter hoc fallacy.
In Hume’s logic, we would have to lump such things as roosters crowing before the sun rises with billiard balls striking each other. Even if we do not understand a particular cause, it is not the case, as Hume explains, that “We have no idea of the connexion, nor even any distinct notion what it is we desire to know, where we endeavour at a conception of it.”
Sure, Hume is correct that when we see an object for the first time and are ignorant of its properties, we are not able to predict what it will do. And he is also correct that we need repeated observations to first learn the connection between the cause and effect that we observe. But once we do, then we are confident about how billiard balls and violin strings cause things because we know how they work. It is not the case that we forever assume the effect follows the cause merely because one customarily follows the other.
I am reminded of a very old TV show called The Beverly Hillbillies. In the show, a hillbilly family who has never seen modern life is transplanted to a mansion in Beverly Hills. One of the running gags is that when the doorbell rings, the family does not connect the sound of music coming from the walls with the fact that someone pushed the front doorbell button. They merely say that whenever that music comes from the walls, soon after someone always knocks at the door. Hume would have us forever stuck in the Beverly Hillbillies show, never realizing how the doorbell works. However, once we repeatedly observe the effect, we can understand how the mechanism works, and we do indeed know how the cause generates the effect.