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Evolutionary Theory and Experience. Popper (1974, p. 1114) argued that our ‘experiences are not only motives for accepting or rejecting an observational statement, but they may even be described as inconclusive reasons’. These reasons are inconclusive reasons for the act of accepting or rejecting an observational statement; they are not inconclusive reasons for or against the content of an observational statement. In arguing for this view, Popper (1974, p. 1112) presented arguments from evolutionary theory. He wrote that sense organs are part of the decoding mechanism by which certain organisms, especially animals, interpret the state of their environment and anticipate its impending changes. They work astonishingly well, although they are far from perfect: they are fallible. ‘They are marvelously powerful and efficient as organs of adaptation; but they are fallible, especially in unfamiliar circumstances.’ Evolutionary epistemology supports the view that perceptual experience often gives us information about the environment, which it is rational to accept as reliable in spite of the fact that this information is in principle fallible (cf. Campbell 1974).
These naturalistic arguments in favour of perceptual experience as a good reason for accepting test statements do not mean that Popper’s epistemology is merely naturalistic. He (1959, § 10) argued against the naturalistic approach to the theory of method: an epistemology or theory of method relying only on naturalistic arguments is uncritical. In spite of this, naturalistic arguments are very important as critical arguments in the discussion of epistemological and methodological problems (cf. Albert 1987, § 14).
It is not circular to use naturalistic arguments in the discussion of epistemological problems. Our critical discussions are not intended to prove any epistemological position. For an epistemology trying to justify positions with the help of sufficient reasons, the charge of circularity would be serious. For an epistemology trying to test positions by using arguments from evolutionary theory, this is not the case.
Epistemic Asymmetry between Test Statements and General Theories. Are there any differences in the critical discussion of general hypotheses and of test statements (cf. Musgrave 1999, p. 342)? Popper (1959, § 30) wrote that there is such a difference: conventions or decisions determine our acceptance of test statements, but not immediately our acceptance of general hypotheses. Musgrave (1999, p. 342) proposed another difference: according to the principle of critical rationalism (CR), it is reasonable to believe non-perceptually in a statement S if and only if S is that statement which has best withstood serious criticism. This principle applies for general hypotheses. For test statements, the principle of experience (E) discussed above applies, saying that it is reasonable perceptually to believe in a statement if and only if it has not failed to withstand criticism. The principle of experience (E) is a concession to the epistemic primacy of sense-experience and of test statements. According to Musgrave, there is an asymmetry in the epistemic situation of general hypotheses and test statements: rationally accepted general hypotheses should have withstood serious criticism, while rationally accepted test statements should not have failed to withstand such criticism. The reason for the weaker requirement for test statements is that we often accept test statements after perception without having tested them with the help of other test statements. According to Musgrave, such test statements are accepted without having withstood serious criticism. They are perceptual beliefs, that is, the belief in them is caused by perceptions.
Experience as a Test. Test statements can be tested by comparing them with other test statements. But they can also be tested by comparing them with experience. Unproblematic test statements about observable events are especially easy to test in this way. We should observe before we accept test statements because observations are tests of the truth-values of the test statements. If we regard perceptual experience as a special kind of test, we need no special principle of experience (E) in order to explain when it is rational to accept test statements. Test statements can be tested directly by experience and can survive such criticism. Take, for example, the test statement ‘There is a planet in position p at time t’. This statement can be tested seriously by observing position p in the sky at time t. If a planet is observed, the test statement has withstood the test. Thus the principle of critical rationalism (CR) can be used also for test statements: it is rational to accept a test statement that has withstood serious criticism. The epistemic asymmetry between general hypotheses and test statements does not consist in the use of two different principles, the principle of experience for test statements and the principle of critical rationalism for general hypotheses. The epistemic asymmetry rather consists in two different kinds of testing. General hypotheses are tested by comparing them with test statements. Test statements can be tested in this way. But they can also be tested by comparing them with experience. Test statements have an epistemic primacy just because they can be directly tested in this second way.
