When the media report on scientific studies, they often tell us science has “proven” that something-or-other is the case. Some of these headlines are quite frankly ridiculous – ranging from “Science Proves That Beer Is The Perfect Human Beverage,” to “Science Proves That You Should Un-Friend Your Ex On Facebook,” and my personal favorite, “Science Proves That Your Cat Hates You.”
These kinds of headlines are, in a sense, obviously ridiculous: No person with half a brain is going to suddenly feel very upset about their relationship with their cat after reading the above headline. It’s fairly clear that the phrase “Science proves that” is being used in an over-the-top sense here, for maximum effect – and that it’s not really proof we’re talking about.
But it’s the more subtle uses of “science proves” that I’m really concerned about. Science obviously can’t prove that your beloved cat secretly despises you. The claim that “Science proves that cannabis prevents cancer tumors,” for example, seems a lot more reasonable.
You can’t handle the proof
But the thing is that, actually, science can’t prove anything. While we all might have learned this in a long-ago science class, it’s easy to forget while reading the news: Science is just not in the business of proving or disproving things in the same way math or logic is. To remember why, let’s look more closely at what we mean by “proof.” A quick Google search gives us the following definition:
proof (noun): evidence or argument establishing a fact or the truth of a statement
There are two important features of the notion of “proof” that make it incompatible with science. The first is the certainty or finality of proof: Once something has been proven, it cannot be disproven (unless a flaw is found in the original proof, in which case it wasn’t really proven in the first place). When a mathematician demonstrates a flawless proof of a theorem, that’s that. No other argument or piece of evidence could possibly show the theorem to be false.
© Uri Bram
In science, on the other hand, even if a piece of evidence points strongly in favor of a certain hypothesis, it’s still possible that another scientist could come along with new evidence pointing in the opposite direction. We have a lot of evidence for the claim that smoking causes lung cancer, for example, so we can be pretty confident in this claim. But scientists haven’t proven that smoking causes lung cancer in the same way mathematicians prove a theorem, because it’s still possible that new evidence will come along. Someone might discover a factor common to almost all smokers which itself is linked to lung cancer, for example. “Smoking causes lung cancer” also doesn’t hold in all situations: It’s a general pattern, but there will still be people who smoke who don’t get lung cancer.
The second feature of “proof” is that it is all or nothing, black and white. In mathematics, a theorem is either proven or unproven – a theorem cannot be partially proven. Science, on the other hand, operates in all shades of grey: We have different levels of evidence for different hypotheses, and therefore different levels of confidence in those hypotheses. There are some scientific hypotheses that we can be pretty confident about: that the sun will rise tomorrow morning, for example. There are others which we have a decent amount of evidence for, and seem pretty uncontroversial – like the smoking and lung cancer example – but are still more likely to be wrong than the “sun rising” hypothesis, and might not hold under all conditions. There are yet other scientific ideas which we have some evidence for, but for which the jury is really still out: whether drinking caffeine is all that harmful, for example.
The job of science is to set out hypotheses – things we think might be true – and then collect evidence to test these hypotheses. If the evidence comes out in favor of our hypothesis, then we can increase our confidence in that hypothesis being true. If the evidence comes out against our hypothesis, then we should decrease our confidence. But since it’s always possible to come across new evidence that might point in either direction, we can never establish that something is true without a shadow of doubt. I can’t even say with certainty that the sun will rise tomorrow morning, or that if I drop my pen it will fall to the floor – even though scientific evidence should make my confidence in these hypotheses very, very high.
The reason that talking about “scientific proof” is so dangerous is that it completely obfuscates all these shades of grey, forcing them to be either black or white. But knowing what shade of grey we’re operating within is essential if we want to make good decisions based on scientific evidence. When a study comes out claiming to prove my cat hates me, I’m not going to take it that seriously. But when a study comes out claiming to prove that cannabis prevents cancer tumors, I might wonder if I should start smoking cannabis. The question we need to ask when we hear these things is not, “Is this true?” but, “How strong is the evidence?” Talking in terms of truth, fact and proof makes it harder for us to think in these terms. But nuance just doesn’t make for sexy headlines – I expect fewer people would have clicked on this piece if I’d titled it “Science Increases Confidence In The Hypothesis That Science Doesn’t Prove Anything…”
Photo: flickr/Tambako The Jaguar