1. Popper (1957 a), p. 198; see also Popper (1975), pp. 82–83 & n.
2. Formally, this is due to the conception of theories as hypothetical statements. Thus, for example, (x) (Fx → (Gx & ¬Gx)) is contradictory only if Fx is assumed true for some x.
3. Popper (1962), pp. 217–218.
4. Popper (1962), pp. 218ff. Note that, as mentioned in Chapter 3, Popper’s notion of testability differs from his notion of severity of test.
5. Popper (1962), p. 220.