This chapter is a critique of German philosopher Rudolf Carnap’s theory on the demarcation between science and metaphysics (i.e., pseudo-science). Carnap argues that whereas science is meaningful, metaphysics is complete nonsense and worthless; the difference lies in that the accuracy of scientific theories can be verified whereas that of metaphysical theories cannot. 

In the introductory remarks of this chapter, Popper briefly describes how he first met Carnap at a seminar around 1928-1929, and they bonded over hiking and sharing ideas. Popper praises Carnap as one of the most captivating people he knows, especially given his readiness to listen to and accept criticism. Having read his recent publications, Popper spends the rest of the chapter discussing how Carnap’s conclusions about the demarcation of science and metaphysics clashes with his own. 

Popper finds Carnap’s theory flawed on two levels. First, he believes that metaphysics, though not science, is not always meaningless. Second, he argues that meaninglessness in itself is not the right marker for distinguishing between science and non-science. It is at once too narrow and too wide a marker: It has been shown to exclude actual scientific theories as meaningless while failing to exclude pseudo-scientific facts. 

Traditionally, philosophers have supported the idea that science is unique for being based on observations and for using an inductive method of reasoning, whereas metaphysics is speculative and abstract.