Karl Popper wrote mostly about the history and method of science. He is generally credited with coining the term “falsification”, arguing that the point of scientific experiments is not to prove or “verify” a theory, but to disprove or “falsify” it.
For example, Einstein’s theories of relativity had great explanatory power: his theories accounted for not only the movements of bodies within observable space, but also for how things like light and gravity functioned on a cosmic scale. Yet even though his theories had been celebrated for years, they had never been experimented on. It was not until the Michelson-Morley experiment that the theories of relativity could really be observed.
Michelson and Morley argued that if Einstein’s theories were true, the stars that you can see near the sun during a total solar eclipse will seem to move. This is because according to Einstein gravity bends light (or space itself, depending on how you look at it). So the sun’s gravity will bend the light coming from distant stars. Einstein himself agreed: if the experiment shows the stars moving, my theories stand; if the stars do not seem to move, my theory is false.
Notice how Einstein did not say “If the stars don’t seem to move, then you guys are cheating!” or, “If the stars don’t move, my theory explains that one too.” Rather, he agreed that his life’s work would be down the tube.
When the experiment showed that the stars did seem to move, Einstein did not claim that his theory was true, only that it stands, for now.
Popper saw this as being a perfect example of how science should work: scientific theories are always provisional, never final. They can never really be proven, only disproven and scrapped in favor of something better.
Karl Popper is best known for applying this approach to what he called pseudo-sciences, theories which claim to be scientific but are not since they cannot be falsified.
The classic example is Marxism. We tend to forget, but the thing that got so many people (especially Western European intellectuals) excited about Marxism was its claim to have discovered the laws of history the same way Newton discovered the laws of nature. Marxism had great explanatory power: and you could interpret all of history, politics, society, and even literature through the Marxist lens of class conflict, superstructure and alienation. To this day, American college kids are encouraged to read Shakespeare’s The Tempest in terms of exploitation of the working class (you know, the magician and Calaban… what? you can’t see it?) as if the Berlin Wall never fell.
But as Popper pointed out, if Marxism really explained everything, it could not be scientific. What might disprove Marxism? Apparently nothing, because even when Marxist predictions about the coming worker’s revolution failed, they could give a Marxist explanation for why their prediction did not come true. No Marxist could ever say “if X event does not happen under Y circumstance, Marxist theory is false.” Therefore Marxism was not science, but something else. It would stumble on in the form of perverse governments or in the hearts of fervent revolutionaries, but it could never really mount an effective reply to Popper’s critique.
We all know people who can explain everything though their pet ideas, and sometimes they act as if their’s is the only intelligent rational, scientific way of looking at things. It probably isn’t. Someone who claims to be scientific without taking into account facts that contradict his theory, or who twists every inconvenient fact in knots to fit his theory is not a scientist, just a blowhard. Likely he is right about a few things, wrong about some others, and completely unaware of how much he gets on your nerves. Just smile nicely and nod.