Is psychology a science? Are the methods used by climate scientists more pseudo-scientific than genuinely scientific? — When we ask questions like these, the issue is “demarcation”: How can we distinguish or demarcate genuine science from non-science?
The word ‘science’ has become such a warm — almost religious — term of approval that practically everyone nowadays is eager to call whatever they do “science”. So hearing the word ‘science’ in a description — especially self-description — of what people do isn’t a reliable indicator that what they do actually is science. When you hear the word ‘science’, beware: there are impostors about.
Followers of Popper say that the mark of science is “falsifiability”: if there is no way a hypothesis can be shown to be false, then it’s not scientific. I hope it’s clear why. We all accept that hypotheses can’t be conclusively verified: a hypothesis is just a guess, and as a guess it can never be proved true the way theorems in mathematics can. But of course a good scientific hypothesis has more going for it than mere guesswork. It “sticks its neck out” by saying something about the real world. If it gets it wrong, the hope is that it will be exposed as false; and if it hasn’t been so exposed, at least not so far, that counts in its favour. But in order to have that count in its favour, it has to be able to stick its neck out. If it can’t do that, followers of Popper think, it can’t be regarded as a scientific hypothesis.
I think these followers of Popper are on to something important, but they’re not quite there yet. Why not? — Because of “holism”, hypotheses can never be decisively falsified either. Hypotheses are held by individual people, along with all the other stuff they believe, which are also hypotheses. Any particular hypothesis which is subject to testing — and therefore potential falsification — is tested alongside innumerable other hypotheses. Together, they imply something that can be observed, and that something either is observed or else it isn’t observed — it’s a “1 or 0 outcome”, if you like.
Followers of Popper want to say that if that something isn’t observed as predicted, then the hypothesis that yielded the prediction is falsified. So even though we have to reject it, it was a scientific hypothesis to begin with; if we had been lucky enough not to have to reject it, so much the better. But things are not as simple is this. Any of the hypotheses, plural, that went into generating the prediction might be singled out as the culprit. With a bit of judicious weeding and planting, we can tend our “garden” so as to keep what we want. For example, my belief in the steady-state theory of the universe yields the prediction that no red shift should be observed in distant astronomical objects. When it is in fact observed, contrary to my prediction, am I obliged to declare my hypothesis falsified? — No, I am not. I can simply make up a new hypothesis, to the effect that light gets “tired” when it travels for very long periods of time and loses energy. This loss of energy manifests itself as decreasing frequency.
In fact, any favoured hypothesis can be protected from the threat of unfavourable observations like that. So any hypothesis can be held in such a way as to make it practically unfalsifiable. So we can’t appeal to the falsifiability or otherwise of a hypothesis as the mark or standard of its being genuinely “scientific”.
It doesn’t follow that no such standard is possible. But instead of focusing on the falsifiability of hypotheses, we should consider instead the way hypotheses are held. If peripheral “excuses” are habitually made so that a hypothesis is held with such tenacity that it is in effect unfalsifiable, what is held is no longer a scientific hypothesis but an ideology. Hypotheses that are made up for the sole purpose of protecting another, favoured hypothesis are called ad hoc hypotheses. Anyone who cares about science should be constantly on the look-out for too great a willingness to make up ad hoc hypotheses. Eagerness of that sort is the mark of ideology rather than science. People in the grip of an ideology can believe almost anything they like, as long as they are prepared to bend over backwards far enough to accommodate their central hypothesis — the one they like.
Followers of Popper who emphasise falsifiability aren’t far wrong, though. What they get right, I think, is the importance of passing tests — by which is meant the honest prediction of otherwise unexpected results, followed by actual observation of the unexpected results. It emphatically does not mean the mere fitting of hypotheses or models to data that have already been gathered and don’t give rise to any sense of surprise.
For a hypothesis to be tested, it must have predictive power. The hypothesis purports to describe things that can’t be observed directly, but it implies things that can be observed, which wouldn’t been anticipated but for the hypothesis yielding its prediction, and so which without it would seem surprising.
Perhaps even more important than predictive power is explanatory power. A hypothesis that has great explanatory power enjoys a logically similar position to one that has great predictive power, with “bafflement” taking the place of “surprise”. In both cases, something that baffles us (in the case of explanation) or would otherwise surprise us (in the case of prediction) is “newly encompassed”. In both cases, what is newly encompassed is implied by a description of some hidden aspect of reality. Such “drawing back of the curtain on reality” is the magic of science, really, and its ability to do that rightly commands respect and often deserves belief. Anything that doesn’t succeed at that feat — whatever masquerades as science but has no real predictive or explanatory power — doesn’t have a legitimate claim to be believed.