Karl Popper is rightly esteemed by those of a liberal persuasion for his many contributions to the cause. In The Open Society and its Enemies – volume 1 he took issue with Plato. He showed that although Plato claimed to be seeking just and virtuous rulers, his Republic resembled the totalitarian warrior state of Sparta with detailed control over every aspect of its citizens' lives. The point, said Popper, was not to choose the best rulers, but to prevent bad or incompetent rulers from doing too much damage.
In volume 2 of The Open Society he took issue first with Hegel, then with Marx, showing that their ideas must lead to coercion and that they contain the roots of totalitarianism, a theme he developed in The Poverty of Historicism, where he countered the socialists' claim that history is moving in their direction toward an inevitable goal.
A strong case could be made that Popper's greatest contribution was the methodology he identified in The Logic of Scientific Discovery. He showed that science does not proceed by deducing new knowledge, or by induction from the past and present to the future, but by conjecturing theories that are then tested in experiments. The imagination leaps forward beyond present knowledge to suggest something new, and the consequences that would follow from the new thing are then tested in the real world to see if they accord with it.
Popper devised the notion of falsification. We do not test theories to see if we can prove them to be true; we test them to see if we can prove them false. Popper saw an asymmetry, in that no amount of experimentation will prove a theory to be true, because a new test might go against it tomorrow. On the other hand, Popper thought that we could show a theory to be false by an experiment that does indeed go against it.
Popper thought that by eliminating known falsehoods we could be left with an ever more concentrated core of truths, of theories that have survived attempts to disprove them. He spoke of "Objective Knowledge," and wrote a book under that title. He thought that there were truths about the universe that are true whether or not we recognise them as such. There was a pool of knowledge to be gleaned about the universe, and by using his methodology of conjecture and refutation, we could gain access to more of it.
In my book Trial and Error I adopted Popper's methodology but took issue with his metaphysic. My problem was that I do not think things can be proved to be false any more than they can be proved to be true. In the first case if we prove something to be false we are thereby proving its converse to be true. It is also the case that in an experiment we are assuming that our senses are not deceiving us when we make the observation. We are also relying on what we already know to test whether it is compatible or incompatible with the new theory. We are not testing for truth but for consistency.
I don't think science uncovers truths about the universe. Its aim is to predict what we shall observe, and it discards the theories that do not help us to do that, and retains the ones that do. It is not that the theories are true of false, but about whether they help us to achieve out purpose – to predict what we shall observe. The ones that do that are useful to us, the others are not.
We want to predict what we shall observe because we want to control it. We want to control our circumstance and render ourselves less vulnerable to fate and fortune. Some animals grow warm coats in winter; we build central heating. Other animals adapt to their environment; we adapt the environment. We have evolved over millions of years with an inbuilt drive to control our circumstance and to cope with whatever chance might throw our way. It is how we survive.
Our theories are inspired guesses that might explain the world we observe, and we test them against each other to see which ones do it better. We reject those shown to be less effective at prediction, and we retain the ones that are more effective. We are not proving some of them false, merely discarding them because they serve our purposes less well than the others, the ones that survive the process of experiment. We keep those because they are useful to us.
Our theories may look like true or false descriptions of what the universe is actually like, but in reality they are more like mental models of it; and we test them to see which ones successfully mirror the world of our observation. In this terminology Sir Isaac Newton did not discover the theory of gravity; he invented it. He constructed a mental model that enabled him to predict the behaviour of rolling balls on Earth and the orbits of the planets in space. It was not a truth waiting to be discovered, but something he created in his mind to help him and others to explain present observations and to predict future ones.
Instead of proving or disproving theories, we are retaining or rejecting them. We are not doing so because they represent objective knowledge, but because they serve our purposes. We call them useful or non-useful to the degree to which they do that. We call the method inspired trial and error, but we are not proving error, only choosing to discard inferior attempts to achieve our purposes.
This method mirrors to some extent the way evolution works. The variations less well equipped to survive are counted out by a selective death rate. The major difference is that the innovations produced by evolution come at random, whereas the new theories we devise have intelligent minds producing them. This means that our ability to increase our control over circumstance is faster than any advantages produced by evolution. It took evolution millions of years to produce human beings; it took those human beings only 12,000 years to crawl out of their caves and plant their footprints on the moon.
For the last word on the subject I go to Xenophanes, a philosopher who wrote 2,500 years ago.
“The Ethiops say that their gods are flat-nosed and black,
While the Thracians say that theirs have blue eyes and red hair.
Yet if cattle or horses or lions had hands and could draw,
And could sculpt like men, then the horses would draw their gods
Like horses, and cattle like cattle; and each they would shape
Bodies of gods in the likeness, each kind, of their own.”
“The gods did not reveal, from the beginning,
All things to us, but in the course of time
Through seeking we may learn and know things better.
These things are, we conjecture, like the truth.
But as for certain truth, no man has known it,
Nor will he, neither of the gods
Nor yet of all the things of which I speak.
For even if by chance he were to utter
The final truth, he would himself not know it:
For all is but a woven web of guesses.”