world, or facts that can be induced from "derived experience". This is referred to as the "verifiability" criterion. This distinction between science, which in the view of the Vienna Circle possessed empirically verifiable statements, and what they pejoratively called "metaphysics", which lacked such statements, can be seen as representing another aspect of the demarcation problem. Logical positivism is often discussed in the context of the demarcation between science and non-science or pseudoscience. However, "The verificationist proposals had the aim of solving a distinctly different demarcation problem, namely that between science and metaphysics." Falsifiability Karl Popper saw demarcation as a central problem in the philosophy of science. Popper articulates the problem of demarcation as: Falsifiability is the demarcation criterion proposed by Popper as opposed to verificationism: "statements or systems of statements, in order to be ranked as scientific, must be capable of conflicting with possible, or conceivable observations". Against verifiability Popper rejected solutions to the problem of demarcation that are grounded in inductive reasoning, and so rejected logical-positivist responses to the problem of demarcation. He argued that logical-positivists want to create a demarcation between the metaphysical and the empirical because they believe that empirical claims are meaningful and metaphysical ones are not. Unlike the Vienna Circle, Popper stated that his proposal was not a criterion of "meaningfulness". Popper argued that the Humean induction problem shows that there is no way to make meaningful universal statements on the basis of any number of empirical observations. Therefore, empirical statements are no more "verifiable" than metaphysical statements. This creates a problem for the line of demarcation the positivists wanted to cleave between the empirical and the metaphysical. By their very own "verifiability criterion", Popper argued, the empirical is subsumed into the metaphysical, and the line of demarcation between the two becomes non-existent. The solution of falsifiability In Popper's later work, he stated that falsifiability is both a necessary and sufficient criterion for demarcation. He described falsifiability as a property of "the logical structure of sentences and classes of sentences", so that a statement's scientific or non-scientific status does not change over time. This has been summarized as a statement being falsifiable "if and only if it logically contradicts some (empirical) sentence that describes a logically possible event that it would be logically possible to observe". Kuhnian postpositivism Thomas Kuhn, an American historian and philosopher of science, is often connected with what has been called