Against logical positivism, Russell thinks that to defend the very possibility of objective knowledge it is necessary to permit knowledge to rest in part on non-empirical propositions. In Inquiry into Meaning and Truth (1940) and Human Knowledge: Its Scope and Limits (1948) Russell views the claim that all knowledge is derived from experience as self-refuting and hence inadequate to a theory of knowledge: as David Hume showed, empiricism uses principles of reason that cannot be proved by experience. Specifically, inductive reasoning about experience presupposes that the future will resemble the past, but this belief or principle cannot similarly be proved by induction from experience without incurring a vicious circle. Russell is therefore willing to accept induction as involving a non-empirical logical principle, since, without it, science is impossible. He thus continues to hold that there are general principles, comprised of universals, which we know a priori. Russell affirms the existence of general non-empirical propositions on the grounds, for example, that the incompatibility of red/blue is neither logical nor a generalization from experience (Inquiry, p. 82). Finally, against the logical positivists, Russell rejects the verificationist principle that propositions are true or false only if they are verifiable, and he rejects the idea that propositions make sense only if they are empirically verifiable.