This is the the eleventh post in my Theories of Knowledge and Reality series. Follow the link for more on the series and for links to other entries as they appear.
David Hume argues that we can't know anything about scientific laws or laws of nature. He, like Descartes, considers what's immediately before our senses. From that, how do I get information about what will happen if I hold my pen out and let go? How do I know it will fall? I may remember it falling in the past. Does that mean it will continue to fall in future cases just like past ones? Even if it's fallen lots of times in the past, how can I be sure it will continue to do so? We can revise scientific laws because we had ignored facts (as Einstein showed that there was data Newton hadn't account for, and as Copernicus showed there was data Ptolemy hadn't accounted for). But Hume argues that one fact will never be available to us -- the future. Will nature always be as orderly as it has been in the past? What if in 3 seconds everything will go haywire? Can we rule that out? Knowing scientific laws requires trusting that nature is uniform. Hume gives us reason not to be sure about that. What gives us reason to think science is better than studying your belly button at discovering truths about the world?
Hume's weaker worry is that the future might not be like the past. Maybe there are scientific laws that have been true but will change tomorrow. How can we rule that out? If knowledge requires certainty, it seems we can’t know anything by doing science. The stronger worry is that there may be no laws at all. Maybe there's nothing making nature uniform, and we just got lucky that things have seemed uniform to us during the short time we've been around. But if nothing's making it this way, why couldn't it change? If you really wanted to take Hume's views to their required conclusion, you'd even have to question whether we know what happened in the past to begin with. If all we have access to is what immediately appears to us to be true at the moment, we can't even be sure of regularities in the past. Hume himself occasionally recognizes this problem, but it isn't the one he dwells on. Even assuming knowledge of the past, he has doubts about knowledge that gravity will work in the future.
I want to look at two kinds of responses that students often come up with immediately upon hearing this problem to show why they completely miss the point. I'll deal with responses that have a little more to them in the next post.
The "It Works" Response: Why is science a good method of attaining beliefs, while navel-gazing isn't? Some argue that science gives good results, while belly buttons aren't so reliable. We do know that science has given good results in the past (assuming, as Hume most of the time does, that our memories are reliable), and belly buttons may not have given reliable results in the past. But to go beyond that is assuming what you are trying to prove. We know that it’s been reliable in the past. What we're trying to prove that it will continue to be reliable in the future. The fact that it's been reliable in the past won’t convince Hume that it will be reliable in the future. It assumes what we're trying to prove. Philosophers calls this kind of circular argument begging the question. It relies on the conclusion to prove that very conclusion. The conclusion we want to prove is that the future will be like the past in terms of scientific laws. It's begging the question to assume we have a good method in the future by relying on a method that was good in the past. If you try to show that scientific methods in the future will be good on the ground that they've been good in the past, then you're assuming that the future will be like the past in exactly the way that's in question. So this response has no hope.
Probability Response: Some people respond that we may not know scientific laws, but it's still likely that certain scientific laws are true. We don't know for sure that they're true, but they're probable given our evidence. We know no more than that, but at least that's something. Science will at least give us probable laws even if it doesn't give us knowledge of laws. That seems fine until we investigate what 'probable' might mean here. We often say something is probable if it occurs frequently. Rolling two sixes is improbable because it doesn't happen often. But here we can say only that it has occurred frequently in the past, not that it will occur frequently in the future. We don’t have any knowledge about how frequently it will occur, and isn't it important to know how frequently it will occur overall? Sometimes we say it's probable when we mean that it's reasonable to believe things, but isn't that begging the question again? It relies on the very thing we want to prove. We're trying to show that it's reasonable to believe scientific laws. You can't just assert that it's reasonable in order to prove that it's reasonable. So these accounts of probability don't seem very successful to motivate the response that scientific laws are probable. Is there some other sort of probability that will work? I haven't encountered one.
So these two responses don't seem to help very much. They both assume exactly the thing we want to prove, so the skeptic won't be very convinced, because the skeptic is questioning exactly the thing these responses rely on to establish their point. In the next post, I'll move on to how some of the general approaches to responding to skepticism can appear in this particular kind of skepticism.