Popper, trying to demarcate empirical science and nonscience, developed the falsifiability thesis as the criterion for demarcation, a criterion used for demarcating empirical science from metaphysics too. Popper, in opposition to Logical Positivists' meaningfulness criterion, claimed that metaphysics may be meaningful, while it is not a science. Calling a set of currently untestable ideas as metaphysical, he claimed that they may be testable in future, so that we may regard a set of metaphysical ideas as protoscience. He afterwards propounded “ metaphysical research programme” as historical development of a science out of a metaphysical idea. Applying the criterion of testability to demarcating science and metaphysics allows some currently metaphysical ideas to become “testable” and, therefore, “scientific”. Thus, Popper's conception of “metaphysics” is different from the original conception of the term. In spite of the importance of regarding metaphysical statements as meaningful by Popper, his particular conception of metaphysics leads to a confusion of metaphysics with protoscience, and gives rise to serious difficulties in philosophy, science, history of science, and philosophy of science.