While Logical Positivism and, later, Scientific Realism were developing their concepts of verification and the idea that scientific knowledge is the actual truth of the world, another philosophy of science was evolving, defending the opposite views. Pragmatism, formulated in the end of the 19th century by Charles Pierce and continued by William James and John Dewey, building on the earlier philosophy of Utilitarianism, postulates that everything must have a purpose, and the purpose is the most important property. The purpose of the scientific research is to reduce doubt, and it does so by combining and applying repetitively abduction, deduction, and induction in a cycle of inquiry. An important notion introduced by Pierce was fallibilism, the idea that all knowledge could, in principle, be mistaken, because the empiric element could always be revised by new observations. The conclusions of science are always tentative. The rationality of the scientific method does not depend on the certainty of its conclusions, but on its self-corrective character.
This notion caught on and evolved into the entire philosophy of Critical Rationalism, built in the 20th century by Karl Popper (1902-1994), and continued by William Bartley, Hans Albert, David Miller, and many others, becoming the leading philosophy of science by the beginning of the 21st century. Popper rejected empiricism, induction, and everything that grew out of it. He pointed out, instead, that no number of positive expermental results can ever absolutely confirm a scientific theory, but a single counterexample can show it to be false. That property of theories, falsifiability, is what he chose to be the criterion of science: a theory is only scientific if it is falsifiable. It may be counter-intuitive, but this is how he explains the growth of scientific knowledge. In response to a given problem, scientists come up with a large number of conjectures, or hypotheses. These hypotheses undergo rigorous attemps at refutation, eliminating most of them. The ones that survive this selection are not true, since nothing, under this philosophy, can be true, but they are the most fit, in the same sense as the living organisms are the ones most fit to the environment after millennia of natural selection that killed off everything less fit. These surviving theories are the ones that work best for the initial problem and are, under this philosophy, verisimilar, or truth-like.