Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single counterexample is logically decisive: it shows the theory, from which the implication is derived, to be false. To say that a given statement (e.g., the statement of a law of some scientific theory) -- [call it "T"] -- is "falsifiable" does not mean that "T" is false. Rather, it means that, if "T" is false, then (in principle), "T" could be shown to be false, by observation or by experiment. Popper's account of the logical asymmetry between verification and falsifiability lies at the heart of his philosophy of science. It also inspired him to take falsifiability as his criterion of demarcation between what is, and is not, genuinely scientific: a theory should be considered scientific if, and only if, it is falsifiable. This led him to attack the claims of both psychoanalysis and contemporary Marxism to scientific status, on the basis that their theories are not falsifiable.

Popper pointed out an important logical asymmetry between what two concepts?