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9
KARL POPPER
The Problem of Induction
A scientist, whether theorist or experimenter, puts forward statements, or systems of statements, and tests them step by step. In the field of the empirical sciences, more particularly, he constructs hypotheses, or systems of theories, and tests them against experience by observation and experiment. I suggest that it is the task of the logic of scientific discovery, or the logic of knowledge, to give a

The Logic of Discuvery (London: Hutchinson, 1959), pp. 27-33.
Reprinted with permission.

logical analysis of this procedure; that is, to analyze the method of the empirical sciences.
But what are these 'methods of the empirical sciences'? And what do we call 'empirical science'?

THE PROBLEM OF INDUCTION
According to a widely accepted view-to be opposed in this book-the empirical sciences can be characterized by the fact that they use 'inductive methods', as they are called. According to this view, the logic of scientific discovery would be identical

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PART 2 I Ioduction and Confirmation: The Nature of Scientific Inference

with inductive logic, i.e., with the logical analysis of these inductive methods.
It is usual to call an inference 'inductive' if it passes from singular statements (sometimes also called 'particular' statements), such as accounts of the results of observations or experiments, to universal statements, such as hypotheses or theories.
Now it is far from obvious, from a logical point of view, that we are justified in inferring universal state-

ments from singular ones, no matter how numerous; for any conclusion drawn in this way may always turn out to be false: no matter how many instances of white swans we may have observed, this does not justify the conclusion that all swans are white.
The question whether inductive inferences are justified, or under what conditions, is known as the

problem of induction.
The problem of induction may also he formulated as the question of how to establish the truth of universal statements which are based on experience, such as the hypotheses and theoretical systems of the empirical sciences. For many people believe that the truth of these universal statements is 'kno'llm by experience'; yet it is clear that an account of an experience-of an observation or the result of an experirnent--can in the fIrst place be only a singular statement and not a universal one. Accordingly, people who say of a universal statement that we know its truth from experience usually mean that the truth of this universal statement can somehow be reduced to the truth of singular ones, and that these singular ones are known by experience to be true; which amounts to saying that the universal statement is based on inductive inference. Thus to ask whether there are natural laws known to be true appears to be only another way of asking whether inductive inferences are logically justified.
Yet if we want to find a way of justifying inductive inferences, we must ftrst of all try to establish a principle ofinduction. A principle of induction would be a statement with the help of which we could put inductive inferences into a logically acceptable form In the eyes of the upholders of inductive logic, a principle of induction is of supreme importance

for scientific method: ' ... this principle', says
Reichenbach, 'deterntines the truth of scientific theories. To eliminate it from science would mean nothing less than to deprive science of the power to decide the truth or falsity of its theories. Without it,

clearly, science would no longer have the right to distinguish its theories from the fanciful and arbi-

trary creations of the poet's mind.' 1
Now this principle of induction cannot be a purely logical truth like a tautology or an analytic statement. Indeed, if there were such a thing as a purely logical principle of induction, there would be no problem of induction; for in this case, all inductive inferences would have to be regarded as purely logical or