"A theory is falsifiable, as we saw in section 23, if there exists at least one non-empty class of homotypic basic statements which are forbidden by it; that is, if the class of its potential falsifiers is not empty."
Popper, The Logic of Scientific Discovery, p. 95
"He asserted that if a statement is to be scientific rather than metaphysical it must be falsifiable [...]. He then based his philosophy of science on the hypothetico-deductive method, claiming that enumerative INDUCTION is invalid, and indeed does not in fact occur, while verification and CONFIRMATION (as opposed to his own ‘corroboration’) are impossible. As his philosophy of science said we should aim to eliminate the false rather than establish the true, [...]."
Lacey (1996)
"A falsifiable hypothesis is one which can be put to a test by which it could conceivably be refuted. The concept is important in Karl Popper’s philosophy of science, according to which the distinctive feature of any scientific theory is that its hypotheses can be put to a test. The distinctive feature of a good scientific theory is that its hypotheses pass the test. The contrast is with pseudo-science. The adherents of a pseudo-science are able to cling to its hypotheses no matter how events turn out, because the hypotheses are not testable."
Mautner (2000)
"Popper’s first major contribution to philosophy was his novel solution to the problem of the demarcation of science. According to the time-honoured view, science, properly so called, is distinguished by its inductive method (see INDUCTION) – by its characteristic use of observation and experiment, as opposed to purely logical analysis, to establish its results. The great difficulty was that no run of favourable observational data, however long and unbroken, is logically sufficient to establish the truth of an unrestricted generalization. This led immediately to the disquiet but apparently inescapable conclusion that science (or at least the important part of it that deals in such generalizations) simply had to live by faith in some kind of UNIFORMITY OF NATURE, hard to define satisfactorily, and seemingly impossible to prove without circularity.
Popper accepted that unrestricted generalizations could not be verified. But, he pointed out, they could be falsified. (While no amount of observation of black crows verifies the statement ‘All crows are black’, one – properly authenicated — observation of a white crow falsifies it.) And falsifiability, for Popper, is the hallmark of science. Science, in other words, characteristically puts itself at risk, commits itself, by implication at least, as to what is, or would be, observed under specific circumstances; and hence its theories are always liable to be discarded or modified if the observations fail to agree with its expectations. It follows that no scientific theory is ever conclusively verified, no matter how many tests it has survived. And this conclusion, Popper points out, accords very well with the history of science: even something as well attested and widely accepted as Newtonian physics has not proved permanently immune from revision."
Flew and Priest (2002)
Problems with Falsificationism
From Chalmers (1999):
Problems stemming from the logical situation
"When observation and experiment provide evidence that conflicts with the predictions of some law or theory, it may be the evidence which is at fault rather than the law or theory."
"A realistic scientific theory will consist of a complex of universal statements rather than a single statement like “All swans are white”. Further, if a theory is to be experimentally tested, then more will be involved than those statements that constitute the theory under test. The theory will need to be augmented by auxiliary assumptions, such as laws and theories governing the use of any instruments used, for instance. In addition, in order to deduce some prediction the validity of which is to be experimentally tested, it will be necessary to add initial conditions such as a description of the experimental set-up."
Falsification inadequate on historical grounds
"In the early years of its life, Newton’s gravitational theory was falsified by observations of the moon’s orbit"
"A second example concerns Bohr’s theory of the atom, and is due to Lakatos (1970, pp. 140-54)."
"A third example concerns the kinetic theory and has the advantage that the falsification of that theory at birth was explicitly acknowledged by its originator."
"A fourth example, the Copernican Revolution, will be outlined in more detail in the following section."
From Ladyman (2002):
Some legitimate parts of science seem not to be falsifiable
Probabilistic statements
Existential statements "Although Popper is right that a universal generalization can be falsified by just one negative instance, many statements in science are not of this form. For example, scientific theories assert the existence of things like black holes, atoms, viruses, DNA and so on. Statements that assert the existence of something cannot be falsified by one’s failure to find them."
Unfalsifiable scientific principles "the principle of conservation of energy", "the second law of thermodynamics", there is no ‘action at a distance’ e.g. "Newton’s theory of gravity, the superiority of simple and unifying theories
Hypothesis of natural selection
Falsificationism is not itself falsifiable
The notion of degree of falsifiability is problematic
Popper cannot account for our expectations about the future
Scientists sometimes ignore falsification
From O'Hear (1989):
"First, many empirically provable statements, which we would intuitively think of as scientific, become unscientific. Thus, ‘There is at least one planet’, ‘There are electrons’, and ‘Bacteria exist‘, are all unscientific, because unfalsifiable. We cannot disprove them by observation, in other words."
"The second immediate problem concerns probability, which plays an increasingly important role in science. A probability statement is one which says that a particular proportion of events will be of such and such a character, but without specifying which ones. Thus, we can say that a certain coin has a 1 in 2 probability of coming down heads (p(h) = 0.5). The problem with such a statement is that it cannot be falsified if no limit is put on the possible number of coin tosses. 10,000 tails in succession would not strictly refute p(h) = 0.5, because over a very long run of tosses 10,000 tails might be balanced out by a large population of heads, and this could be said of any deviation at all from any predicted probability."
"A key feature of the objectivity of science is the repeatability of observations and experiments. Insisting on repeatability guards against observer bias and inaccuracy, to say nothing of dishonesty, and against freak results due to chance or unusual factors interfering with a particular observation. As such it is a crucial aspect of the objectivity and openness of science, but it means that theories are falsified not by single observations or experiments, but by what is in effect another general hypothesis to the effect that such and such an observation is repeatable."
"There is always an implicit assumption in any test situation that there is no factor interfering with the observed result so as materially to affect the result.
Conclusion
Popper's 'scientific method' requires an infinite number of hypotheses, yet is not robust, fails with existential statements, fails with probabilistic statements and fails in practice anyway due to the necessity of auxiliary assumptions. How has Popper's falsification performed in practice? Newton's gravitational theory, Bohr's theory of the atom, kinetic theory, the Copernican Revolution and the theory of evolution were all falsified, despite being excellent examples of science. Popper's falsification fails in both theory and practice.