Section I: from justification to criticism: the unfinished project of critical rationalism

As argued earlier, the general separation between justification and criticism has played a significant role in various stages during the development of the idea of critical rationalism. Recognizing that the justified true belief account of knowledge is not consistent with the practice of science. Popper applied the notion of 'conjectural knowledge' to present a non-justificationist theory of science. The general separation between justification and criticism thus found its first expression in what Bartley (1964) called Popper's non-justificational philosophy of criticism.

Popper’s critical rationalism and the separation between justification and criticism

Realizing that justification is superfluous for the existence of scientific knowledge, Popper (1992a [1959]) argues that the Humean problem of induction means that empirical evidence cannot justify the truth of a universal law. By separating justification and criticism. Popper was able to argue that consideration of objective knowledge merely as unfalsified conjecture and not as justified true belief solves the problem of induction because the logic of science shows how to use experience to refute a deductively formulated scientific claim, not how to justify the conclusion of a scientific deduction by its premises.

Popper (1962) discovered that the reinvention of the logic of science is taken place by replacing the inductive method with the method of conjecture and refutation using the modus tollens form of inference. Popper (1992b) claims that modus tollens refers to a non-justificationist form of criticism according to which the rule of retransmission of falsity from the conclusion to the premises facilitates refutation of a scientific theory by means of the possibility of a false prediction. Ignoring the fact that modus tollens justifies the falsity of one premise for refutation of the prediction. Popper supposes that modus tollens provides the logic of science with a non-justificationist form of criticism whose purpose is not to justify a universal conclusion by limited sense experiences. However, Popper is wrong to say that modus tollens falsifies a universal law by even only one piece of counterevidence because of the justifying role that even this one piece of evidence plays in such refutation.

Nevertheless, Popper's usage of the separation between justification and criticism led to the more convincing deductive theory of empirical science as a replacement for the inductive theory of science. Popper realizes that the logical form of modus tollens is not applicable for a non-justificationist examination of metaphysical theories simply because there is no empirical evidence to refute the conclusion of a metaphysical theory. The form of modus ponens, on the other hand, is useless for metaphysics due the undisputable premises in its justification of the tnith of the conclusion. Hence, Popper (1963) offers the notion of the problem-solving ability of a metaphysical theory to show that a non-empirical theory can also be criticized despite that the standard of problem-solving does not offer a ‘logical form of criticism’ for relating the truth or falsity of the conclusion to the truth or falsity of the premises.

Instead of applying his idea of the problem-solving ability of a philosophical theory to see how a claim of rationality might be true. Popper offers a justificational concept of critical rationalism in terms of irrational faith in reason. Instead of pursuing the separation between justification and criticism to demarcate ‘rational’ from ‘irrational’ belief, Popper takes a justificationist turn, arguing that the inability of the rationalist to justify all of his positions by argument or experience also leads to his inability to justify his own belief in critical rationalism. Hence, a critical rationalist should confess a minimum level of irrationalism.