A Demarcation between Science and Non-Science:
His criterion concerns, not a theory’s confirmability, but instead its falsifiability, that is, its susceptibility to be proven incorrect.
Marx… Freud… Adler…
Their theories have apparent explanatory power. These theories appeared to be able to explain practically everything that happened within the fields to which they referred. Once your eyes were thus opened, you saw confirming instances everywhere: the world was full of verifications of their theories. (Human Being’s Confirmation Bias)
(1) It is easy to obtain confirmations, or verifications, for nearly every theory-if we look for confirmations.
(2) Confirmations should count only if they are the result of risky predictions; that is to say, if, unenlightened by the theory in question, we should have expected an event which was incompatible with the theory--an event which would have refuted the theory.
(3) Every 'good' scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
(4) A theory which is not refutable by any conceivable event is nonscientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.
(5) Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability: some theories are more testable, more exposed to refutation, than others; they take, as it were, greater risks.
(6) Confirming evidence should not count except when it is the result of a genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory. (I now speak in such cases of 'corroborating evidence'.)
(7) Some genuinely testable theories, when found to be false, are still upheld by their admirers - for example by introducing ad hoc some auxiliary assumption, or by re-interpreting the theory ad hoc in such a way that it escapes refutation. Such a procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status. (I later described such a rescuing operation as a 'conventionalist twist' or a 'conventionalist stratagem’.)
One can sum up all this by saying that the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.
About Hume…
I found Hume's refutation of inductive inference clear and conclusive. But I felt completely dissatisfied with his psychological explanation of induction in terms of custom or habit.
Without waiting, passively, for repetitions to impress or impose regularities upon us, we actively try to impose regularities upon the world. We try to discover similarities in it, and to interpret it in terms of laws invented by us. Without waiting for premises we jump to conclusions.
This was a theory of trial and error - of conjectures and refutations.
But the distinction between dogmatic and critical thinking, or the dogmatic and the critical attitude, brings us right back to our central problem. For the dogmatic attitude is clearly related to the tendency to verify our laws and schemata by seeking to apply them and to confirm them, even to the point of neglecting refutations, whereas the critical attitude is one of readiness to change them - to test them; to refute them; to falsify them, if possible. This suggests that we may identify the critical attitude with the scientific attitude, and the dogmatic attitude with the one which we have described as pseudo-scientific.
Popper has summarized some of his conclusions as follows:
(1) Induction, i.e. inference based on many observations, is a myth. It is neither a psychological fact, nor a fact of ordinary life, nor one of scientific procedure.
(2) The actual procedure of science is to operate with conjectures: to jump to conclusions-- often after one single observation (as noticed for example by Hume and Born).
(3) Repeated observations and experiments function in science as tests of our conjectures or hypotheses, i.e. as attempted refutations.
(4) The mistaken belief in induction is fortified by the need for a criterion of demarcation which, it is traditionally but wrongly believed, only the inductive method can provide.
(5) The conception of such an inductive method, like the criterion of verifiability, implies a faulty demarcation.
(6) None of this is altered in the least if we say that induction makes theories only probable rather than certain.
Bibliography
Popper, Karl R. (1957). Philosophy of science: A personal report. In J. H. Muirhead (ed.), British Philosophy in the Mid-Century. George Allen and Unwin. pp. 182--83.