Critical rationalism

"Critical rationalism" is the name sometimes given to the philosophy that Karl Popper created and that's how I use it here. Popper wrote extensively about many philosophical problems and I have no chance of summarising here everything that he said. Instead, I have focussed on some of his key ideas.

Demarcation and induction

Popper was born on the 28th of July 1902 in Vienna, which at that time was part of the Austro-Hungarian Empire, and he died on the 17th of September 1994. At the start of his career, in the 1920s and 1930s, Popper was seeking to solve two main problems, namely the problem of demarcation and the problem of induction. He solved the problem of how scientific statements differ from metaphysical ones by stipulating that empirical statements had to be falsifiable. He differed from the logical positivists in many ways; one of which was that he did not think that metaphysical statements were meaningless. He solved the problem of induction by denying that induction existed.

The traditional scientific method

Induction is a key component of the traditional scientific method. According to this science begins with the collection of observations; when enough of these have been amassed a theory is formulated and then this is verified. Rejecting the existence of induction, Popper had to provide an alternative account of how science develops. Not only did he deny the existence of induction, he also argued that science does not begin with observation and that universal scientific theories cannot be verified.

Popper's account of how science grows

For Popper science begins with problems, an example of which is "How do the planets move?" (Over the centuries, several different theories have been proposed as solutions to this problem and I look at some of them elsewhere.) Troubled by a problem, a scientist guesses at a solution. It doesn't matter where the theory he proposes comes from; there is no rational method of arriving at scientific theories. Having come up with a theory, the scientist should then try his hardest to falsify that theory. The theories that survive intense criticism then become part of scientific knowledge, but this status is always provisional. Just because a theory has withstood serious attempts to falsify it, that doesn't mean that a successful falsification isn't just around the corner.

Popper's account of the development of science is an idealisation to some extent: science should begin with problems and scientists should try to falsify their theories. In practice, some scientists do collect observations without a clear problem in mind and others strongly defend their pet theories as they view their falsification as a disaster. For Popper, the falsification of a theory is always a good thing as it shows that science is making progress and it encourages scientists to come up with better explanations of the phenomena they are interested in.

Popper's problem-solving schema

In his later writings Popper summarised his account of how science progresses by means of the following tetradic problem-solving schema:


Here, P1 is the initial problem; TT is a tentative theory put forward to solve P1; EE is the process of error elimination in which TT is thoroughly criticised; and P2 is a problem thrown up by the error-elimination process. (This schema occurs frequently in his later writings; it is found, for example, in Objective Knowledge on pp. 119, 121, 126, 144, 164, 168, 243, 287 and 297.)

Although originally proposed as an account of the growth of science, Popper extended the application of his problem-solving schema to include, not only all intellectual activity, but also practical matters. Thus, P1 can be either a practical problem or a theoretical problem. He gives the following example of his schema in action when P1 is a practical problem (from Knowledge and the Body-mind Problem, p. 11):

Henry Ford's original problem was: how can we provide transport for the vast spaces of the United States? This was his P1. He proposed the theory: by building a cheap motor car. This led through various trials and errors to a new problem: how can we provide the roads and parking places needed for your cars? The original problem P1 was the problem of transport. The new problem P2 is the traffic problem—a problem of frustration.

Popper's influence

Popper's ideas on the growth of scientific knowledge have been tremendously influential, not only within philosophy, but also amongst scientists and academics in other disciplines. Amongst the philosophers he influenced are Paul Feyerabend, Imre Lakatos, William Warren Bartley, III, Joseph Agassi, David Miller and Ian Jarvie. Amongst scientists, he influenced the biologist Peter Medawar and the neurophysiologist John Eccles. Others he influenced include the art historian Ernst Gombrich and the investor George Soros.


  • Karl Popper, Objective Knowledge: An Evolutionary Approach, London, Oxford University Press, 1972.
  • Karl Popper, Knowledge and the Body-mind Problem: In Defence of Interaction, London, Routledge, 1994.

© Antoni Diller (1 April 2014)