by Charlie Huenemann

Philosophy students are typically taught the wrong lesson from the great Scottish skeptic David Hume. The standard story goes something like this. British empiricists like Locke and Berkeley wanted to connect everything we know to what we experience through the senses. The welcome consequence of this strategy is that all the stuff we see and interact with stays known – but the spooky invisible stuff, ranging from magical spirits to substantial forms and other metaphysical clutter, all goes by the wayside. But (the story continues) Hume pointed out that this strategy ends up far more corrosive than anyone expected: for, if we hold our beliefs to what we actually experience, we shall have no knowledge of causality. We see one event, and another; but never do we experience the metaphysical glue that connects the two, and forces the second event to follow the first.

The take-away lesson is that, according to Hume, we really have no knowledge of causality, and – if we are rational – we should be completely surprised every time we strike a match. This of course seems utterly loony, and it leads to spirited classroom arguments (which by itself, I’ll allow, is a good reason to teach Hume this way). How could it possibly be right that the fully rational person would not see causality at work in the world?

Well, it isn’t; and in truth, Hume never thought it was. As he defended himself to an incredulous correspondent,

… I never asserted so absurd a Proposition as that any thing might arise without a Cause: I only maintain’d, that our Certainty of the Falsehood of that Proposition proceeded neither from Intuition [sense experience] nor Demonstration; but from another Source.

Hume wasn’t a skeptic about causality. He only maintained that the causal knowledge we have does not arise from our sense experience or from our reasoning.

What’s the difference? It turns out to be an interesting one. In the first (wrong) story, the lesson is that there is no such thing as causality. That’s certainly a bold claim, but it’s not in the least compelling. No one can take it seriously except as some kind of trivial philosophical nut to crack. In the second (correct) story, the lesson is that human knowledge is not as straightforward as philosophers would like. What we know does not boil down to rational inferences from observations and arguments. It’s more natural, more organic, than that.

Hume’s own answer to where causal knowledge comes from (that other “Source” he mentions) is not itself anything mysterious. It is custom or habit, the same sort of thing that got Pavlov’s dogs to salivate when they heard a bell. We become shaped by the patterns of the world, used to the world’s regularities, and that shaping is what makes us expect the world to continue on as it has in the past. But, of course, this is no argument for believing the world really will continue to behave itself – “past performance does not guarantee future results,” as our stockbrokers warn us – and that is precisely Hume’s point. We believe it will, we’re even stubbornly certain of it – we know it! – and meanwhile we have no decent argument to lean back on. Funny thing, this human knowledge: our minds are more like those of dogs than like that of Socrates.

Now some smarty pants might present a subtle objection to Hume’s identifying custom or habit as the source of our causal knowledge. For what is custom or habit? It is a causal process of our minds getting shaped by the patterns of the world. But we are trying to explain what prompts us to believe in causality in the first place. Hume’s answer seems to be “Well, causality does!” while at the same time denying that we have any good reason to believe in causality. Isn’t he helping himself to something he has yet to earn?

But the objection fails to take in the grand theme of Hume’s philosophy, which is to show that our everyday knowledge of familiar things is always more compelling than any philosophical (let alone religious) explanation. Our grasp of causality, in particular, is far more sure than our comprehension of alleged metaphysical explanations that are supposed to account for it. The level-headed person gives priority to what is known familiarly, and doubts our flights of philosophical fancy.

I find Hume’s view interesting because it makes me wonder what it might mean to be Humean today. Those of us who follow the headlines issuing from the world of science feel like, on the whole, humans are getting amazingly good at discerning the hidden springs and principles of nature. We don’t know everything, of course – but we do seem to know a whopping lot about the existence and behavior of subatomic particles, the effects of relativity, the Higgsian nature of mass, and on and on. But these are just the sort of tenuous, abstractly metaphysical accounts of nature Hume thought we could never really believe with full confidence, especially when (as in the whole of particle physics) they describe strange behaviors and properties out of keeping with how the macroscopic world works. So how would a true Humean regard today’s theoretical accounts of the world?

On the one hand, we might say that nothing has changed. After all, our scientific knowledge still relies on causality, or at least statistical regularities, and Hume’s arguments on that score are as pertinent today as they were then. We have fancier examples and more impressive claims to knowledge, but in the end they fall prey to the same skeptical arguments. Hume’s arguments are timeless.

On the other hand, Hume’s point was that our practical, everyday world is better known to us than anything the metaphysicians rave on about, even those metaphysicians who publish on arXiv. Today’s Hume might well narrow his eyes at such publications, saying, “Well, maybe so, and maybe no; there is a whole lot of inferring going on there, and plenty of opportunity for missteps. Methinks your reach is exceeding your grasp.” But to this many of us would reply, “You’re wrong there, Davy. We’re not just spinning out stories in the way your metaphysicians did. We test our conjectures with experiments, carefully check our calculations and our results, and we try to trim the explanations as narrowly as we can. And – by the way – it all seems to check out rather well!” In short, there is a decent worry that Hume’s skeptical stance was predicated on a relatively primitive state of science, a state in which metaphysical speculation was only loosely tethered to experiment. It made sense to be Humean in the 18th century and earlier, but not so much today.

The question is over the acreage of the proper Humean epistemic fortress. Do we create a tidy castle of common sense, and protect ourselves against all those who challenge it, whether they be religious zealots, philosophers, or physicists? Or do we allow for a larger, more capacious walled city, letting in all things well-documented by experiment, and warily keeping out untested speculations? (Bad news, String Theory; you’ll be waiting outside for some time either way!)