I have come to the conclusion that Darwinism is not a testable scientific theory, but a metaphysical research programme – a possible framework for testable scientific theories (p.167)
I do not think that Darwinism can explain the origin of life. I think it quite possible that life is so extremely improbably that nothing can “explain” why it originated; for statistical explanation must operate, in the last instance, with very high probabilities. But if our high probabilities are merely low probabilities which have become high because of the immensity of the available time (as in Boltzmann’s “explanation”), then we must not forget that in this way it is possible to “explain” almost everything. Even so, we have little enough reason to conjecture that any explanation of this sort is applicable to the origin of life. But this does not affect the view of Darwinism as situational logic, once life and its framework are assumed to constitute our “situation”.
I think that there is more to say for Darwinism than that it is just one metaphysical research programme among others. Indeed, its close resemblance to situational logic may account for its great success, in spite of the almost tautological character inherent in the Darwinian formulation of it, and for the fact that so far no serious competitor has come forward. (p. 168)
(Karl Popper, “Darwinism as a metaphysical research programme”, cited in Michael Ruse, Philosophy after Darwin: classic and contemporary readings, Princeton University Press, 2009)