The method of trial and error is applied not only by Einstein but, in a more dogmatic fashion, by the amoeba also.
— Popper (1968, 68)
For Popper (1968), ‘the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability’ (48) by means of observations. ‘Thus science must begin with myths, and with the criticism of myths’ (66); ‘we may point out that every statement involves interpretation in the light of theories, and that it is therefore uncertain’ (55n.). ‘To put it more concisely, similarity-for-us is the product of a response involving interpretations (which may be inadequate) and anticipations or expectations (which may never be fulfilled)’ (59). Thus ‘repetition-for-us’ is ‘the result of our propensity to expect regularities and to search for them’ (60).
Observation is always selective. It needs a chosen object, a definite task, an interest, a point of view, a problem. And its description presupposes a descriptive language, with property words; it presupposes similarity and classification, which in their turn presuppose interests, points of view, and problems.
There is no measurement without a theory and no operation which can be satisfactorily described in non-theoretical terms.
— Popper 1968 (61, 82)
Most of the beliefs which actually guide practice, or determine one’s path, are not falsifiable in the way that would qualify them for ‘scientific status.’ Indeed it is doubtful whether any theory outside of the special sciences is falsifiable in that way. For Popper, such enterprises as Freudian psychoanalysis or Marxist dialectical materialism were not sciences but quasi-religions. Peirce had much the same attitude toward the kind of ‘psychical research’ current his day; yet he considered philosophy a science, at least potentially. To do otherwise would block the road of inquiry, which would be even worse than being too credulous.