These worlds can be described as follows (though they should be read from the bottom starting at World 1, and then upwards to be meaningful):
World 3: both (i) unembodied objects such as abstract entities (and, most probably, universals), mathematical entities, social institutions and objective knowledge, and (ii) embodied entities* that are products of human design, engineering and production such as buildings, books, clothes, money, and art works;World 1 is the most basic, and Worlds 2 and Worlds 3 are higher-level worlds that emerge from World 1.
↑World 2: the world of conscious minds with intentionality and mental states;
↑World 1: the spatiotemporal world of physical objects and events, matter and energy and their relations, and even biological systems considered merely as complex physical systems.
* “Embodied entities” are members of both World 3 and World 1, but in different senses: e.g., considered purely physically as spatiotemporal objects describable in scientific terms, they belong to World 1; but when considered as human cultural artefacts, they belong to World 3.
While the entities in World 2 that we call minds are causally dependent on World 1 and could not exist without World 1, they are also objectively real and new ontological entities and emergent phenomena (Niiniluoto 2006: 60).
Entities in World 3 are causally dependent on human minds (or at least some kind of conscious mind).
They are constructed by the mind and could not exist without the conscious minds of World 2 (Niiniluoto 2006: 61). But, although entities in World 3 are created by minds in World 2, they are also autonomous and objective once we have created them by thought and continue to think of them. They can also have unintended consequences and lead to downwards causation, as human minds apply or discover new ways of using World 3 entities, such as, for example, in scientific theories in changing and shaping World 1. Therefore Worlds 3 and 2 can affect World 1, so that causation runs both upwards and downwards. (For instance, free will, if humans really do have it, is a World 2 phenomenon.)
Popper’s three world ontology has the following virtues:
(1) it avoids the fallacy of strong reductionism;Jesper Jespersen (2009) uses Popper’s three worlds ontology in his Critical Realist methodology for Post Keynesian economics, which is influenced by Tony Lawson, and which is also more or less adopted by other Post Keynesians such as Philip Arestis, Victoria Chick, and Sheila Dow (Jespersen 2009: 57).
(2) it is compatible with a moderate realist or conceptualist view of abstract entities and universals;
(3) it understands that human minds are an important, real entity in the universe and is consistent with John Searle’s biological naturalist theory of the mind, even though minds are a type of emergent physical property (for why Searle is not a property dualist, see Searle 2002);
(4) it is compatible with the finding of modern science that emergent properties are a fundamental phenomenon in our universe;
(5) it is compatible with downwards causation;
(6) it is ultimately consistent with the view that higher-level worlds are causally dependent for their existence on lower-level worlds.
“Popper’s Three Worlds,” Wikipedia
Edward Feser, “Popper’s World 3,” July 26, 2010
Anderson, P. W. 1972. “More Is Different,” Science n.s. 177.4047: 393–396.
Jespersen, Jesper. 2009. Macroeconomic Methodology: A Post-Keynesian Perspective. Edward Elgar, Cheltenham, Glos. and Northampton, MA.
Niiniluoto, Ilkka. 2006. “World 3: A Critical Defence,” in Ian Jarvie, Karl Milford, and David Miller (eds.), Karl Popper: A Centenary Assessment. Volume II. Metaphysics and Epistemology. Ashgate, Aldershot. 59–69.
Popper, Karl R. 1978. “Three Worlds,” The Tanner Lecture on Human Values, Delivered at the University of Michigan, April 7, 1978.
Popper, Karl R. 1979. Objective Knowledge: An Evolutionary Approach (rev. edn.). Clarendon Press, Oxford.
Searle, J. 2002. “Why I Am Not a Property Dualist,” Journal of Consciousness Studies 9.12: 57–64.