In Early Modern Philosophy we have come to Hume’s critique of causality (Enquiry, section 4). I have always had difficulty getting the nature of his critique straight in my mind. So I sat down to try to put it clearly, and came up with the following. Nothing new or original here – just what I hope is a clear articulation of one of Hume’s great insights.
We all know what happens in the story that begins, “David Hume came to a fork in the road….” The two tines of the fork are marked “relations of ideas” and “matters of fact,” which Hume regards as “the only two objects of human reason.” What makes a true relation of ideas true is the fact that when you try to deny it, you contradict yourself. What makes a true matter of fact true is … a good question. Hume would like to find an answer to it. He observes that every purported matter of fact is founded in one way or another upon cause and effect. From what we immediately perceive we infer a cause; from our memories, we infer a past event; from our past experience of two events being always conjoined, we infer the second from evidence of the first. And so on. So, he claims, if we want to know what makes true matters of fact true, we need to know what makes causal claims true. Hume then proceeds to demonstrate that general causal claims are not relations of ideas, since if you deny a causal claim, you will not thereby contradict yourself (though you will run the risk of sounding silly). But neither are general causal claims matters of fact, since … well, since why? Remember, we are trying to figure out what makes true matters of fact true. We don’t know the answer yet.
Hmm. Well, practically, what Hume ends up assuming is that a true matter of fact is true in virtue of accurately capturing what is present in our experience. So I eat some bread, and it nourishes me. I eat some more, and it nourishes me, too. The true matter of fact issuing from this experience is that the bread I have eaten on these two occasions has nourished me. But typically we also infer something much stronger: that the bread that I will eat in the future also will nourish me, or that bread always has nourishing qualities (under similar conditions). But these claims do not accurately capture what is present in my experience, if only for the reason that the future, let alone what “always” happens, is not present in my experience.
If this is enough to disqualify general causal claims as matters of fact, then Hume has succeeded in showing that causal claims are not objects of human reason. What does that mean? It does not mean that we should be skeptical of causal claims. It means we should not take ourselves to have any real understanding of why they are true. As he writes towards the end of part I of section 4:
Elasticity, gravity, cohesion of parts, communication of motion by impulse; these are probably the ultimate causes and principles which we discover in nature; and we may esteem ourselves sufficiently happy, if, by accurate inquiry and reasoning, we can trace up the particular phenomena to, or near to, these general principles. The most perfect philosophy staves off our ignorance a little longer: as perhaps the most perfect philosophy of the moral or metaphysical kind serves only to discover to larger portions of it [namely, our ignorance]. Thus the observation of human blindness and weakness is the result of all philosophy, and meets us at every turn, in spite of our efforts to elude it or avoid it.