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Three Worlds

Popper, Karl. 1979. Three Worlds. Ann Arbor,: University of Michigan. https://www.thee-online.com/Documents/Popper-3Worlds.pdf.

Karl Raimund Popper was born in Vienna in 1902. He studied at the University of Vienna, where he received his Ph.D. in 1928. He lectured in Canterbury, New Zealand, from 1937 to 1945 and in the London School of Economics (University of London) from 1946 to 1969. Sir Karl is a Fellow of the Royal Society and of the British Academy and a member of several national and international academies.

Reading level: Intermediate


I would guess that most people, when they take the time to consider how many worlds they live in, would answer “two”. These would be the physical and mental worlds. There is some sort of relationship between what is “out there” and how we sense it and then represent it on our minds. That relationship is complex and subjective. There is also some question about whether we exist in a duality, with mind and body being entirely separate, or in some sort of unified whole where mind and body are one. I suspect that research into brain science may give rise to some new forms of philosophy shortly, or at least some new questions and arguments.

One of the enduring philosophical questions is whether and how the mind can have causal effect on the “real world”. in his 1978 lecture on human values,  Karl Popper  delves into these questions and adds a new wrinkle. He suggests that we live in three worlds, rather than two.

Popper’s three worlds are as follows:

  1. The physical world, perhaps subdivided into non-living and living, e.g. rocks and trees.
  2. The mental or psychological world, comprised of individual human experiences.
  3. The world of products of the human mind, such as language, stories, art and institutions.

These three worlds correspond to the notion of objective (1), subjective (2) and inter-subjective (3).

Okay, so this is not too big a stretch, until you ask the question about which of these three worlds is real? Popper argues that since both of the mental worlds (human minds and their products) can have causal effects on physical things, both are real. He differentiates between subjective and objective knowledge.

Knowledge in the subjective sense consists of concrete mental dispositions especially of expectations; it consists of concrete world 2 thought processes, with their correlated world 1 brain processes. It may be described as our subjective world of expectations. Knowledge in the objective sense consists not of thought processes but of thought contents. It is the objective thought content of a conjecture or theory on which the scientist’s subjective thought processes work. Thought contents are, we may conjecture, products of human language; and human languages, in their turn, are the most important and basic of world 3 objects. If I am right that the physical world has been changed by the world 3 products of the human mind, acting through the intervention of the human mind then this means that the worlds 1, 2, and 3, can interact and, therefore, that none of them is causally closed.

So, the next time you drive to the airport and get on a plane, consider this. Roads, cars, airports and airplanes all have physical form and are real. But the meaning and use of roads, cars, airports and airplanes (world 1) arises from (and is caused by) thought contents in world 3, which were themselves generated, understood and agreed to by thought processes in world 2.

Popper’s Three Worlds lecture is available here.