Karl Popper: Theory

“There are all kinds of sources of our knowledge; but none has authority … The fundamental mistake made by the philosophical theory of the ultimate sources of our knowledge is that it does not distinguish clearly enough between questions of origin and questions of validity.”

Karl Popper

Conjectures and Refutations: The Growth of Scientific Knowledge (1963)

Question , Knowledge

“If we are uncritical we shall always find what we want: we shall look for, and find, confirmations, and we shall look away from, and not see, whatever might be dangerous to our pet theories.”

Karl Popper book The Poverty of Historicism

The Poverty of Historicism (1957) Ch. 29 The Unity of Method
Context: If we are uncritical we shall always find what we want: we shall look for, and find, confirmations, and we shall look away from, and not see, whatever might be dangerous to our pet theories. In this way it is only too easy to obtain what appears to be overwhelming evidence in favor of a theory which, if approached critically, would have been refuted.

“I could not think of any human behaviour which could not be interpreted in terms of either theory. It was precisely this fact — that they always fitted, that they were always confirmed — which in the eyes of their admirers constituted the strongest argument in favour of these theories. It began to dawn on me that this apparent strength was in fact their weakness.”

Karl Popper

Conjectures and Refutations: The Growth of Scientific Knowledge (1963)
Context: I may illustrate this by two very different examples of human behaviour: that of a man who pushes a child into the water with the intention of drowning it; and that of a man who sacrifices his life in an attempt to save the child. Each of these two cases can be explained with equal ease in Freudian and in Adlerian terms. According to Freud the first man suffered from repression (say, of some component of his Oedipus complex), while the second man had achieved sublimation. According to Adler the first man suffered from feelings of inferiority (producing perhaps the need to prove to himself that he dared to commit some crime), and so did the second man (whose need was to prove to himself that he dared to rescue the child). I could not think of any human behaviour which could not be interpreted in terms of either theory. It was precisely this fact — that they always fitted, that they were always confirmed — which in the eyes of their admirers constituted the strongest argument in favour of these theories. It began to dawn on me that this apparent strength was in fact their weakness.

Thinking

“Put in a nut-shell, my thesis amounts to this. The repeated attempts made by Rudolf Carnap to show that the demarcation between science and metaphysics coincides with that between sense and nonsense have failed. The reason is that the positivistic concept of 'meaning' or 'sense' (or of verifiability, or of inductive confirmability, etc.) is inappropriate for achieving this demarcation — simply because metaphysics need not be meaningless even though it is not science. In all its variations demarcation by meaninglessness has tended to be at the same time too narrow and too wide: as against all intentions and all claims, it has tended to exclude scientific theories as meaningless, while failing to exclude even that part of metaphysics which is known as 'rational theology'.”

Karl Popper

Ch 11. "The Demarcation between Science and Metaphysics." (Summary, p. 253)
Conjectures and Refutations: The Growth of Scientific Knowledge (1963)

Sense , Science , Time , Parting

“Darwinism is not a testable scientific theory, but a metaphysical research program.”

Karl Popper

Unsourced variant: Evolution is not a fact. Evolution doesn't even qualify as a theory or as a hypothesis. It is a metaphysical research program, and it is not really testable science.
Popper later retracted his criticisms:
I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation.
:* "Natural selection and the emergence of mind" dialectica http://www.dialectica.ch/ Vol. 32 (1978), p. 339-355; republished in Evolutionary Epistemology, Rationality, and the Sociology of Knowledge (1987) edited by Gerard Radnitzky and W. W. Bartley, III
Unended Quest: An Intellectual Autobiography (1976)

“Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.”

Karl Popper

Objective Knowledge: An Evolutionary Approach (1972)

Problem , Appearance

“Good tests kill flawed theories; we remain alive to guess again.”

Karl Popper

As quoted in My Universe : A Transcendent Reality (2011) by Alex Vary, Part II

“As for Adler, I was much impressed by a personal experience. Once, in 1919, I reported to him a case which to me did not seem particularly Adlerian, but which he found no difficulty in analyzing in terms of his theory of inferiority feelings, although he had not even seen the child. Slightly shocked, I asked him how he could be so sure. "Because of my thousandfold experience," he replied; whereupon I could not help saying: "And with this new case, I suppose, your experience has become thousand-and-one-fold."”

Karl Popper

Conjectures and Refutations: The Growth of Scientific Knowledge (1963)

Feeling , Help , Personality

“Scientists try to eliminate their false theories, they try to let them die in their stead. The believer—whether animal or man—perishes with his false beliefs.”

Karl Popper

Source: Epistemology Without A Knowing Subject (1967)

Animals , Falseness