 Popper accordingly rejects the view that induction is the characteristic method of scientific investigation and inference, substituting falsifiability in its place. It is easy, he argues, to obtain evidence in favour of virtually any theory, and he consequently holds that such “corroboration”, as he terms it, should count scientifically only if it is the positive result of a genuinely “risky” prediction, which might conceivably have been false. In a critical sense, Popper’s theory of demarcation is based upon his perception of the asymmetry which, at the level of logic, holds between verification and falsification: it is logically impossible to verify a universal proposition by reference to experience (as Hume saw clearly), but a single genuine counter-instance falsifies the corresponding universal law. In a word, an exception, far from “proving” a rule, conclusively refutes it.