We might think that telling the difference between science and pseudoscience is easy: science is based on empirical evidence, pseudoscience is not. But things are a lot more complicated than that. Coming up with a definition of either science or pseudoscience turns out to be incredibly hard. Instead of looking at the content of the theories to tell whether they count as science, an alternative is to focus on the people putting them forward, the care with which they approach their claims, and their character, writes Massimo Pigliucci.

What’s so difficult about separating sound science from pseudoscience? Let’s give it a try. Fundamental physics is, clearly, science. Homeopathy is, just as clearly, pseudoscience. Why? Well, because the first one is based on empirically well-established theories that make precise and reliable predictions about how the world works, while the second one is founded on notions that contradict basic chemistry and biology and has been demonstrated over and over not to work. Case closed, right?

Not so fast. Take one particular area of fundamental physics: string theory. There has been increasing controversy over the past several years about whether it is, in fact, sound science [1,2]. After all, string theory began to be articulated in the late 1960s. To this day, more than half a century later, it has not been able to provide a single empirically testable prediction. And a growing number of scientists are beginning to think that the theory, with its “landscape” of 10^500 possible solutions, is in fact impossible to test in principle. Does that make it a pseudoscience? Not quite.