Karl Popper: Science and Falsifiability

The problem which troubled me…was [that] I wished to distinguish between science and pseudo-science [non-science or false science], knowing very well that science often errs, and that pseudo-science may happen to stumble on the truth.

I know, of course, the most widely accepted answer to my problem: that science is distinguished from pseudo-science…by its empirical method, which is essentially inductive, proceeding from observation or experiment. But this did not satisfy me. I often formulated my problem as one of distinguishing between a genuinely empirical method and a non-empirical or even a pseudo-empirical method—that is to say, a method which, although it appeals to observation and experiment, nevertheless does not come up to scientific standards. The latter method may be exemplified by astrology, with its stupendous mass of empirical evidence based on observation—on horoscopes and on biographies.

I found that those of my friends who were admirers of Marx, Freud and Adler, were impressed by a number of points common to these theories, and especially by their apparent explanatory power. These theories appeared to be able to explain practically everything that happened within the fields to which they referred…. The world was full of verification of the theory. Whatever happened always confirmed it….

… A Marxist could not open a newspaper without finding on every page confirming evidence for his interpretation of history … The Freudian analysts emphasized that their theories were constantly verified by their “clinical observations.”…

With Einstein’s theory the situation was strikingly different. Take one typical instance—Einstein’s prediction … that light must be attracted by heavy bodies. [This was confirmed by Eddington’s expedition, which measured the shift in the light coming from a star.]

Now the impressive thing about this case is the risk involved in a prediction of this kind.

These considerations led me in the winter of 1919-20 to conclusions which may now reformulate as follows.

• It is easy to obtain confirmations, or verifications, for nearly every theory—if we look for confirmations.
• Confirmations should count only if they are the result of risky predictions…
• Every “good” scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
• A theory which is not refutable by any conceivable event is non-scientific….
• Every genuine test of a theory is an attempt to falsify it ….
• Confirming evidence should not count except when it is the result of a genuine test of the theory….

The belief that science proceeds from observation to theory is still so widely and so firmly held that my denial of it is often met with incredulity….

But in fact the belief that we can start with pure observations alone, without anything in the nature of a theory, is absurd; as may be illustrated by the story of the man who dedicated his life to natural science, wrote down everything he could observe, and bequeathed his priceless collection of observations to the Royal Society to be used as inductive evidence. This story should show us that though beetles may profitably be collected, observations may not.

… I tried to bring home the same point to a group of physics students in Vienna by beginning a lecture with the following instructions: “Take pencil and paper, carefully observe, and write down what you have observed!” They asked, of course, what I wanted them to observe. Clearly the instruction, “Observe!” is absurd…. Observation is always selective. It needs a chosen object, a definite task, an interest, a point of view, a problem.

[Karl Popper, “Science: Conjectures and Refutation.” In British Philosopher in Mid-Century, ed. By C.A. Mace, 1957. London: Routledge, 1963 (3^rd ed. 1969), ch.1.]