Popper's three worlds

Popper's three worlds is a theory developed by Karl Popper in the late 1960s. It involves three interacting worlds. World 1 is the material realm, World 2 is the mental realm, and World 3 is the cultural realm. Popper's goal was to defend his notion of objective knowledge against the rising notion that knowledge is a belief that must be justified and true. This theory supported his old view that theories need not be verified by induction. In his approach, the methodological rules as well as the logical content of science belong to World 3. The theory is evolutionary. Popper was a strong advocate of a theory of emergence in which each world is not predetermined by previous ones.

Objective knowledge and inductionism Popper introduced his notion of objective knowledge, essential in world 3, because testing theories is important in science and requires the intersubjectivity of knowledge. For Popper, "objective" did not imply "true" or "certain". On the contrary, for Popper, one value of objective knowledge is that it can be uncertain, progress and get closer to the truth through bold conjectures and their criticisms. Already at the time of his major work in philosophy of science, Logik der Forschung (1934), Popper rejected that scientific knowledge or its falsification had to be rigorously justified from observation. In the context of the fallibility of science, both of its laws and of their falsification, Popper distinguished rigorous logic from non-rigorous methodology and was thus able to discuss the role of deductive logic in science. From the work of Russell in the early 20th century to that of Lakatos in the late 1960s, including that of Carnap and many others, empirical truth (valid correspondence with reality) has played a central role in distinguishing science from non-science. These works attempted to use inductive principles to logically evaluate laws or research programs from observations. Many approaches have been tried and then abandoned. Popper argued that the role of observations in science lay not in logical justifications or induction, but in the evaluation or criticism of bold conjectures, in a methodology that has for goal the search for "truth." This methodology consists of rules, implicit conventions, that guide the overall scientific process. In falsificationism, this scientific process includes the introduction of auxiliary hypotheses which, to justify a claimed falsification, reject possibilities not considered in the initial condition. Popper was aware that the rules governing this process can hardly be rigorous, as they require problematic methodological decisions due to the Duhem–Quine thesis and other practical problems, while the logical aspect is rigorous. Popper argued that even though rigorous logic cannot assess the empirical truth of laws as the inductivist research program attempted to do, it is nevertheless a necessary element in science. Already in the 1930s, Popper wrote that the bulk of scientific activity consists of using deductive logic to check the consistency of a theory, compare theories, check their empirical nature (i.e., falsifiability) and, most importantly, test a theory, which is possible only when it is falsifiable. He emphasized that, even when theories are tested against observations, deductive logic is largely used. Already in the 1930s, Popper discussed the distinction between scientific objectivity and subjective conviction. It was only decades later that Popper would refer to scientific knowledge as objective knowledge in World 3, and describe this scientific activity as an interaction between World 2 and World 3 responsible for the growth of objective knowledge.

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